168 



Prof. J. Prestwioh. 



been flooded by outpourings from fissures at successive times from 

 the close of the Miocene down to the (Quaternary period. In Columbia 

 these basaltic rocks have a thickness of from 1000 to 3000 feet, and in 

 parts of Colorado of not less than 4000 feet, and they stretch over a 

 tract some 700 to 800 miles in length by 80 to 150 miles in width, 

 and cover 120,000 to 150,000 square miles of surface. 



Extensive as are the ejections of some volcanoes at the present 

 day, and vast as are some of the individual lava streams, the sum 

 total is small compared with these older extrusions. Lyell instances 

 as amongst the most remarkable of the modern lava streams that 

 formed during the eruption of .Skaptan Jokul, in Iceland, in 1783. 

 He states that it formed two branches of the relative lengths of 50 

 and 45 miles, and of the extreme breadths of from 12 to 15 miles, 

 and of 7. Taking the mean breadth, we have an area of 500 square 

 miles covered by the lava of this eruption, with an ordinary thickness 

 of about 100 feet, and an occasional one, when it filled gorges, of 600 

 feet. 



But we have no means of judging of the single flows of geological 

 times ; we can only take the total areas covered by recent volcanic 

 eruptions, and compare them with the old basaltic outpourings. The 

 two most extensive modern volcanic surfaces are those of Hawaii and 

 Iceland. The thickness of the masses at the centre of eruption pro- 

 bably exceeds, though the average of the whole mass is certainly 

 below, the thickness of the basaltic beds in the cases named above, but 

 the areas covered by the volcanic materials show very different 

 measures. The area of Hawaii is about 3800 square miles and is 

 entirely volcanic, and that of Iceland is 37,800 square miles, of which 

 the volcanic beds are said to occupy about one-third to one-half. At 

 the outside, therefore, the modern eruptions are spread over no area 

 larger than 20,000 square miles, or an area only equal to one-tenth 

 and one-seventh of the old Indian and American basaltic areas. 



Now, if these vast erupted masses had been derived from local 

 molten lakes of moderate size and moderate depths, the extrava- 

 sation must have caused a diminution in their masses which, as the 

 loss could not be made good by drafts from adjoining areas, must 

 necessarily have led to a caving in and subsidence of the crust above 

 these lakes proportionate to the mass of the extravasated matter. 

 But so far from this being the case, the areas of these great basaltic 

 outwellings are almost always areas of elevation. The basaltic area 

 of the Deccan forms vast plateaux which attain a height of between 

 4000 to 5000 feet, and although the intercalated sedimentary strata 

 are mostly of land and freshwater origin, there is reason to believe 

 from the circumstances that on the borders of the same district some 

 of the associated beds contain estuarine remains, that the area was 

 immediately prior to- the eruption not much above the sea-level. 



