48 C. R. VAN HISE 



extrusions a quantity of magma may be emitted which surpasses 

 the entire amount emitted between epochs of regional extru- 

 sions. 



To appreciate the importance of regional extrusions of 

 magma I need only to recall the Tertiary volcanic period, dur- 

 ing which were produced the great lava plateaus, some of them 

 thousands of feet in thickness, in western North America, Great 

 Britain, Iceland, Franz Josef land, New Zealand, Abyssinia, and 

 India. In western North America the area of these volcanics 

 is to be estimated by hundreds of thousands of square miles. 

 The Deccan traps of India are estimated to cover 200,000 

 square miles and for much of this area to be from 2000 feet to 

 6000 feet thick. 1 But the Tertiary volcanics with which we 

 are acquainted are only a remnant of the quantity emitted ; for 

 during Tertiary and post-Tertiary times, the erosion has been 

 stupendous, and a large fraction of the material extruded has 

 been converted into sedimentary rocks by means of the epigene 

 agents. 



While the volcanic rocks of the Tertiary period surpass in 

 quantity the known igneous rocks of any previous period, it by 

 no means follows that previous volcanic extrusions might not 

 have been on a still vaster scale. For the further back we go, 

 the larger is the fraction of the volcanic rocks of any given period 

 which has been converted into sedimentary rocks by the epigene 

 agents, and, furthermore, the proportion of the volcanics of a 

 given period which is buried under sedimentary and igneous 

 rocks ever increases as time passes by, so that but a small frac- 

 tion of the formations bearing extrusives of great age is exposed, 

 and in these formations, as has been seen, the larger parts of the 

 extrusives have been destroyed. 



Moreover, the extrusives are probably but the smaller part 

 of igneous rocks. In another place I have suggested reasons 

 why intrusives are more extensive than extrusives. 2 For the 



1 Geology of India, by R. D. Oldham : 2d ed., Calcutta, 1893, pp. 256, 263. 



2 Earth movements, by C. R. Van Hise : Proc. Wis. Acad. Sci., Arts, and 

 Letters, Vol. XI, 1898, pp. 495-496. 



