74 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



This association of dykes with the out-welling of lava and with the accumulation of deep 

 and extensive volcanic plateaux, is paralleled in other parts of the world. The description 

 by Mr G. T. Clark of the dykes connected with the vast basaltic sheets of the Bombay 

 Presidency corresponds almost exactly with that which I have given of those of this 

 country. The Indian, like the British, examples occur in great numbers, rising through 

 every rock in the district up to the crests of the Ghauts, 4000 feet above the sea. They 

 vary from one or two to 10, 20, 40, and even occasionally 100 or 150 feet in width, 

 and are often many miles in length. They observe a general parallelism in one average 

 direction, and show no perceptible difference in character even when traced up to elevations 

 of 3000 and 4000 feet* 



To this and other areas, where horizontal sheets of basalt cover enormous tracts of 

 country with no great central volcanic cones from which the material could have come, 

 fuller reference will be made in the next division of this paper, which treats of the basalt- 

 plateaux of the British Islands.t 



II. THE VOLCANIC PLATEAUX. 



We have now to consider the structure and history of those volcanic masses which, 

 during Tertiary time, were ejected to the surface within the area of the British Islands, 

 and now remain as extensive basalt-plateaux. Short though the interval has been in a 

 geological sense since these rocks were erupted, it has been long enough to allow of very 

 considerable movements of the ground and of enormous denudation. Hence the 

 superficial records of Tertiary volcanic action have been reduced to a series of broken and 

 isolated fragments. I have already stated that no evidence now remains to show to what 

 extent there were actual superficial outbursts of volcanic material over the rest of the 

 dyke-region of Britain, and the subsequent waste of the surface has been so enormous 

 that various lava-fields may quite possibly have stretched across parts of England and 

 Scotland, from which they have since been wholly stripped off, leaving behind them only 

 that wonderful system of dykes from which their molten materials were supplied. 



There can be little doubt, however, that whether or not other Phlegrean fields 

 extended over portions of the country whence they have since been worn away, the chief 

 volcanic tract lay to the west in a broad and long depression that stretched from the 

 south of Antrim to the Minch. From the southern to the northern limit of the 

 fragmentary lava-fields that remain in this depression is a distance of some 250 miles, 

 and the average breadth of ground within which these lava-fields are preserved may be 

 taken to range from 20 to 50 miles. If, therefore, the sheets of basalt and layers of tuff 



* Quart. Jour. Geol. Soc, xxv. (1869) p. 163. 



t It is interesting to note that in the great paper on Physical Geology already cited, Hopkins considered the question 

 of the outflow of lava from the fissures which he discussed. " If the quantity of fluid matter forced into these fissures," 

 he says, " be more than they can contain, it will, of course, he ejected over the surface; and if this ejection take place 

 from a considerable number of fissures, and over a tolerably even surface, it is easy to conceive the formation of a bed 

 of the ejected matter of moderate and tolerably uniform thickness, and of any extent" (op. cit., p. 71). 



