James Thomson

XIII. On the Geology of the Island of Islay

There are few parts of Scotland of which we know so little of the geological features as of those of the Island of Islay. The only information we have on the subject is to be found in Dr M‘Culloch’s “Highlands and Western Isles of Scotland,” vol. ii., page 249; and by Prof. Nicol, in his “Geology of the North of Scotland.” Dr M‘Culloch deals principally with the topographical aspect of the island, of which he gives a map and several sections. He describes the rocks in the central valley as being made up of limestone, and notes the occurrence of granite boulders on the eastern side, and Prof. Nicol repeats these observations. But as neither of them identified the stratigraphical position of the rocks of Islay, the object of the present communication is to do so, and it will be best accomplished if I shortly notice, in detail, the stratified deposits. In order that their superposition and succession may be somewhat clearly defined, I have made a section of the most important points, which shows in succession the lithological order. I shall begin with the superior deposits on the south-eastern side, and review them downwards and westwards. The southern end of the island is extended seawards in the form of a peninsula, which, from Slochd-mhaol-Doraidh in the north, to the Maol na h-Otha* in the south, is a distance of about seven miles, terminating at the Maol na h-Otha in bold water-worn precipitous cliffs, about 500 feet in height. The This 250-word extract was created in the absence of an abstract

I. On the Occurrence of Species of the Genus Palastræa (M‘Coy) in the Lower Carboniferous Strata of Scotland, with a Description of some New Species and Notices of Varieties

The object of this communication is to offer evidence in favour of the recognition of the structural details distinguishing the genus Palastræa of M‘Coy which has not been accepted by M.M. Milne Edwards and J. Haime in their well-known monograph on the Carboniferous Corals.* The facts now discovered clearly prove the accuracy of Prof. M‘Coys diagnosis of the genus, which belongs to the family Astræinæ. The genus Palastræa was defined by M‘Coy, in the Annals and Magazine of Natural History† and subsequently in Sedgwick and M‘Coys “Palæozoic Rocks and Fossils,” ‡ as follows:—“Corallum forming very large masses; terminal stars from nine lines to an inch and a half in diameter, obscurely pentagonal, bounded by narrow, rounded, or obscurely angular cellular spaces (no simple divisional walls), having from one hundred and seven to one hundred and thirty thin, jagged, radiating lamellæ, which descend to form an oval or circular cup, and one-half of which rise again to form an oval central boss, by the edge being produced into an abrupt pali-form lobe (not distinguishable from the lamellæ); in the centre the lamellæ become indistinctly blended: vertical section shows the uninterrupted passage of the loose vesicular tissue, conforming to the shape of the terminal cup, in gentle curves from star to star; a very small space directly under the centre of each star having the vesicular structure almost transverse: horizontal section shows the alternately long and short radiating lamellæ connected (except in a small space near the centre) by fine transverse vesicular This 250-word extract was created in the absence of an abstract

XVIII. On the Jointed Prismatic Structure in Basaltic Rocks

The views presented in the following paper have in great part been previously brought forward and published on some other occasions, but not hitherto in any such way as to become satisfactorily accessible among geologists. The fundamental ideas and primary observations on which they were based were submitted by the author to the Belfast Natural History and Philosophical Society in a paper read on November 26, 1862, and the subject was shortly afterwards brought under the notice of the British Association at the Newcastle Meeting in 1863, and a brief but complete account of the chief points is to be found printed in the Association’s Report for that year. A fuller account of the author’s views, with his discussion of the prevailing views of others, is printed in the Annual Report of the Belfast Naturalists’ Field Club for 1869-70, as an abstract of a paper read by him to that Society in November, 1869. He was afterwards requested to bring the subject before the Geological Society of Glasgow; and the present paper contains a fuller exposition than any previous one, and includes the discussion of a few facts which have later come under notice. Basaltic Rocks, and rocks generally of the trappean class (which comprises most of those that have originated as volcanic lavas), are very frequently to be found divided into prismatic columns, more or less regular in form, and the columns are sometimes found to be divided into short lengths by cross fractures or surfaces of easy partition,

VI. On Ctenacanthus Hybodoides, Egerton

Many of the spines found in carboniferous strata have been enshrouded in so much obscurity, that any observation which will contribute to their proper classification is of importance to the palæontologist. The form and structure of the spines of Ctenacanthus have been described by Professor Agassiz; but I am not aware that the spines of the species which forms the subject of this communication have been described as constantly or intimately associated with other head-parts, such as might suggest the particular animal to which they belonged. Teeth and spines have received distinct generic names. Our information not sufficing to determine the spines and teeth which belonged to the same animal, I propose to attempt such a determination in the case of Ctenacanthus hybodoides. In the spring of 1869 I was fortunate in obtaining a slab of the Palace Craig ironstone from Carnbroe, near Airdrie, which contained, 1st, a mass of shagreen; 2nd, two spines of Ctenacanthus hybodoides* (Plate II., fig. 1, 1 a, and 1 b); 3rd, a number of the teeth Cladodus mirabilis (Plate III., fig. 1), all evidently in their proper relative positions, the slightly curved lines in which they are disposed suggesting the contour of the mouth; and, lying at a higher level than these, there is exposed, 4th, a fragment of a small spine, the cross section of which gives in outline an oblong sphere, with the posterior side shorter than the other. The anterior face is round and smooth, while the posterior is flat, and This 250-word extract was created in the absence of an abstract

XLIII. On the Geology of the Campbeltown District

Campbeltown Loch is bounded on the north by Knock Scalbert, and on the south-east by Davar island, an isolated rock which stands sentinel-like sheltering the loch from the fury of the south gales. Knock Scalbert terminates in a ridge of basalt and basaltic greenstone, at the south of Kilkousland burying ground. The basaltic columns are exposed at low water and lie horizontally; while the basaltic greenstone is vertical, and contains between the columns partitions of impure hæmatic ironstone, varying from a quarter of an inch to seven inches in thickness. The basaltic greenstone has yielded to the action of the weather, leaving the ironstone standing like the unfilled cells of a honey-comb. Davar island is connected with the mainland by a low belt of sand and gravel, which is fast encroaching upon the channel of the loch, but is under water at high tide. On the south, rises Ben Gollion, nearly 1000 feet high, composed of schistose rock of various kinds. A succession of knolls rises out of the bare moor upon its summit, their interstices being clothed with moss and heather. On the west, at the head of the loch, stands the town of Campbeltown. On the shore, about a mile to the south of Campbeltown, the rock consists of a fine schist of a reddish grey colour, (and, as Prof. Nichol has pointed out*) somewhat lustrous, especially on the planes of lamination, and containing those lenticular masses of quartz, so common in the schistose rocks. Ascending the hill, This 250-word extract was created in the absence of an abstract

II. On the Stratified Rocks of the Shore-line from Clachland Point to the Cock of Arran

There is probably no part of Scotland which has attracted the attention of geologists so much as the Island of Arran, the number and magnitude of the phenomena exhibited in its comparatively limited area accounting for the stream of students of the science which yearly visits its shores. Several of our ablest observers and best geological writers have from time to time recorded the results of their investigations in the Transactions of various learned societies or in separate publications. Although many interesting facts have thus been published, yet the field seems so inexhaustible that each new discovery only seems to stimulate to further research. Having long been one of the band of workers interested in the island, my principal object being the discovery of coralline remains in the Carboniferous rocks, I have, during my visits of investigation there, noted several important facts not previously recorded, and I now propose to bring these forward with some necessary remarks and explanations. Brief mention in detail will be made of the stratified rocks in their geological sequence along the shore-line from Corriegills on the south to the Cock of Arran on the north, with references, so far as practicable, to the palæontological evidence, in order to establish the stratigraphical position of the rocks in an ascending scale. I shall give, in the first place, a list of some of the published papers relating to the geology of that part of the island with which I am at present concerned, though there may be This 250-word extract was created in the absence of an abstract

XV. The Geology of the Territory of Idaho, U.S., and the Silver Lode of Atlanta

The Territory of Idaho is situated on the north-west extremity of the United States of America. It is bounded on the south by Utah, on the west by Oregon, on the north-west by Washington, on the north by British Columbia, on the east by Montana, and on the south-east by Wyoming. Its geological phenomena are varied and interesting. Immediately after crossing the boundary between Idaho and Utah there is an extensive sheet of vesicular lava, which extends for 340 miles across the territory in rolling undulations which resemble Atlantic waves. The long axes of these ridges trend from south to north, and are here and there broken by circular depressions which prove on examination to be the mouths of extinct volcanic craters. Around their margins the lava is covered by a thin vitrified crust with minute undulations, like the ripple marks on a sandy shore, which are deflected in every direction, indicating that the flow of lava had been in some way obstructed. The Snake River, resembling with all its turnings and windings the coils of an enormous serpent, has cut a deep channel, and its escarpments present an unbroken face of vesicular lava. Near the surface the vesicles are minute—in this respect like recent pumicestone—but become larger and more irregular as we descend in the sections. Near the middle of this dreary waste, turning to the east at Mountain Home, the rough pathway lies across the surface of the lava for fully 30 miles. About midway the party crossed

XXVI. Notes on the Metamorphic Rocks of Harris and North Uist

The author first briefly referred to the opinions expressed by some of our most eminent observers regarding the stratigraphical arrangement of the rocks in Inverness-shire, Ross-shire, and Sutherlandshire, and he stated that M‘Culloch and Murchison had described the rocks of the Outer Hebrides as belonging to the fundamental gneiss which has been frequently spoken of by others as the oldest in the world. Mr. Thomson then described the characteristics of the rocks of Harris, including granite, granitoid-gneiss, gneiss, and conglomerate. The latter he had found interstratified with the gneiss and granitoid-gneiss. He referred to a paper by Dr. James Geikie, in the Quarterly Journal of the Geological Society, and said that a more faithful and graphic account of the results of ice-action he had never read; but he was surprised that Dr. Geikie had not spoken of the conglomerates found interstratified with the gneissic rocks. At Loch Maddy, in North Uist, these rocks are much of a similar character to those of Harris, and the conglomerate rocks are seen exposed close to the pier and in front of the inn, and from this point they extend southward. Their contained fragments, pebbles, and boulders, consist of hornblende, gneiss, and granitoid rocks; while the basic rock consists of a dull blue-grey felspathic rock, with numerous patches of vitreous quartz. These erratics do not resemble fragments which had fallen from cliffs, neither do they appear to have been exposed to the action of water upon a coast line, but have a great resemblance

XXIII. On the Stratified Rocks of the Garpel Water, Muirkirk, Ayrshire

In order that my subject may be better understood, I propose to take a preliminary glance at the stratigraphical position of the rocks found in the strath, from Glenbuck on the east to Ballochmyle on the west. At the eastern extremity of Glenbuck, where the water for Catrine Mills is collected, and before it enters the Water of Ayr, the rocks belong to the Upper Silurians. Overlying them is a thin bed of Old Red Sandstone, which again is overlaid by Lower Carboniferous strata, these latter to be seen abutting against, and flanking the Old Red Sandstone all round the lower or western end of Glenbuck. The Carboniferous rocks fill in and form the centre of the valley, and the low blackband ironstone which is seen cropping out on either side has been wrought for a considerable period, and is still being worked at various depths nearly to Auchinleck on the west. In the Greenock Water, on the north side of the valley, the upper rocks are seen overlying breccia, which in turn overlies a series of argillaceous shales, more or less chocolate coloured, with the pale yellowish concretionary limestone which characterises this horizon of Upper Old Red. Similar sections are seen all along the north bank of the Ayr Water, except at Ballochmyle, where there is a great fault. There the deep-red Permian rocks are seen in juxtaposition to the upper limestones of the Scottish coalfields. The dislocation is well seen at Lugar, at which place the rocks can This 250-word extract was created in the absence of an abstract

Abstracts of Papers:— III. On the Geology of Lewis

The authors briefly described the relations of the gneissic or Laurentian rocks to the Cambrian strata of the island. The junction of the two formations is seen in the bed of a small stream that flows into the sea in the harbour of Stornoway; also in Garabost Bay, about seven miles to the east. The Laurentians dip N.W., while the lower members of the Cambrian dip at an angle of 23 deg. to the N.E. The basic rocks partake more or less of a gneissic character, but on the eastern side of the bay are seen passing with imperceptible gradations into a deep, dull, chocolate colour. On the western side there is a precipitous escarpment of conglomerate, in which it is difficult to trace the plane of stratification, but where this can be made out, it is seen to dip to the S.W., or unconformably to the lower members of the series. These beds have been termed by Sir Roderick Murchison, Upper Cambrian. The conglomerates are in two divisions. Near the base of the group the boulders and pebbles are all rounded, similar to those found along the more exposed parts of the West Coast, and consist of gneiss, granitic gneiss, and granite, the majority being of greyish colour. The boulders of the upper conglomerate are all more or less of red binary and ternary granite. Special attention was drawn to this fact, as corroborating the classification of the sections seen in Islay, as described by Mr. Thomson at a This 250-word extract was created in the absence of an abstract