Howel Williams

Pliocene volcanoes of the Navajo-Hopi country

INTRODUCTION Scores of volcanic necks, dikes, and lava-capped mesas rise from the high plateau of northeast Arizona and the adjacent parts of Utah and New Mexico. These are the remnants of a volcanic field that formerly covered many thousands of square miles. Erosion has so far dissected this field that the original cones have disappeared, the sheets of lava have been dismembered, and the old volcanic conduits now rise as giant towers, revealing their inner structure with singular clarity. The objects of this paper are to discuss the features of these denuded volcanoes, the character of their ejecta, and the age of their eruptions. To petrologists, the region is one of unusual interest, because of the similarity of its igneous rocks to certain of those in the classic regions of alkaline rocks in Montana and Wyoming. So extensive is the Navajo-Hopi volcanic province, that the present survey is to be . . .

The Geology of Snowdon, North Wales

I. Introduction. The area to be described is limited on the north-east by the Pass of Llanberis from near the village of Nant Peris to Pen-y-gwrhyd; on the south-east by the Vale of Gwynant down to Beddgelert; on the south-west by the Colwyn and Gwyrfai valleys to near the village of Salem; and on the north-west by the supposed line of junction between the Cambrian and the Ordovician rocks. The approximately quadrilateral area of Ordovician rocks thus defined has an extent of about 30 square miles. It is represented on portions of the 1-inch Ordnance Survey maps 106 & 119, of the 6-inch sheet, Caernarvonshire 22, and of the Quarter-sheets 16 S.E., 17 S.W., 21 N.E. & S.E., 27 N.E., & 28 N.W. It includes parts of the Quarter-sheets 75 N.E. & 78 S.E. of the 1-inch Geological Survey map (old series). The general configuration of Snowdon is represented on the map, PL. XXIX. Prof. W. M. Daviss paper on ‘Glacial Erosion in North Wales’ [42] affords a useful guide to the topography, and is considered adequate for the present purpose. (a) Historical Review. On his first visit to North Wales in 1831, Sedgwick drew a section through Snowdon, in which he depicted the synclinal structure of the summit and the descending sequence of beds towards the north-west. From this section and from others across Y Glyder Fawr and Moel Hebog, he ascertained the general parallelism of the fold-axes in this part of Caernarvonshire. Subsequently, he determined the general succession

Newberry volcano of central Oregon

ACKNOWLEDGMENTS In the preparation of this paper, the writer has been assisted in many ways. It is a pleasure to record thanks due to Cordell Durrell, a graduate student of the University of California, for his able assistance in the field, and for preparing the outlines of the block diagrams (Figs. 3 and 4). Unfortunately, the excellent advance sheet of the Newberry Crater quadrangle was not available for field-work, but the reconnaissance maps of the Forest Service were supplemented, in part, by detailed plans kindly loaned by George Conklin, of the Shevlin-Hixon Lumber Company. The writer is also glad to acknowledge his indebtedness to Professor C. A. Anderson, not only for a critical review of the manuscript, but also for many discussions, all the more welcome in view of the fact that he has lately commenced a detailed study of the Medicine Lake Highlands of northern California, which bear many . . .

Problems and progress in volcanology

Summary Volcanoes are, and always have been, concentrated chiefly along continental edges and within ocean basins. Volcanism is most nearly continuous in orogenic belts, other than those of Alpine-Himalayan type, and in the oceans; it is intermittent and short-lived adjoining orogenic belts and ceases for vey long periods only in continetal nuclei. More information is needed concerning the shapes and depths of volcanic reservoirs. Wedging, stoping and fluxing rather than explosive churning are the chief means by which vents are opened. The source and maintenance of heat are largely unsolved problems, but much of the requisite thermal energy probably comes from ascending juvenile gases. Among the factors that control volcanic behaviour one of the chief is the rate of vesiculation; magma composition, degree of crystallization, retrograde boiling and depth of magma chambers appear to be relatively unimportant. The origin of the water vapour given off by volcanoes remains in dispute, but in certain kinds of eruptions groundwater plays an important part. Many calderas and volcano-tectonic depressions owe their formation chiefly to large-scale subterranean migration of magma. methods of prediction are being improved; seismic and tilt measurements now offer the most reliable information, but magnetic and electric methods merit further trial.

The Geology of the Dolwyddelan Syncline (North Wales)

The importance of the Dolwyddelan syncline from a stratigraphical standpoint has long been recognized. It was considered by Ramsay to be a vital link or ‘key-area’ in the supposed correlation of the Snowdonian country with that of Bala. More recently, it was learned that the syncline contained graptolite-shales intimately connected with the Snowdonian volcanic rocks and offering definite evidence of their age. Considering, further, the unfortunate circumstance that on Snowdon itself the highest lavas are also the topmost beds, and that the lowest lavas are separated by a thick group of sediments from the highest graptolite horizon yet recognized there, the need for a re-study of the Dolwyddelan syncline, especially for more precise evidence as to the age of the Snowdon lavas, becomes apparent. The limits of any syncline are, of course, arbitrary, and as our chief aim has been to examine the volcanic rocks and the sediments immediately associated with them, we have restricted ourselves to the re-mapping of a narrow zone extending for about a third of a mile on either side of the synclinal axis over a distance of some 6 miles. We have, in addition, made a reconnaissance traverse along the southern slopes of Moel Siabod in order to evaluate the relations of the Snowdonian lavas of the syncline to the Capel Curig lavas which underlie them. The area illustrated on the map (Pl. XXXV) is also depicted on the 1-inch Geological Survey map (Old Series), Quarter-sheet 75 N.E., and comprises the 6-inch Quarter-sheet Caernarvon-shire 23