On the Agency of Water in Volcanic Eruptions. 169 



In America also the basaltic plateaux rise gradually to heights of 

 from 2000, 3000, and 4000 feet, and in some cases even attain the 

 height of 11,000 feet or more. Similar evidence, though on a much 

 smaller scale, is afforded by the basaltic plateaux of Ireland and 

 Scotland, of Central France, and of other countries which form rela- 

 tively to the surrounding districts more or less elevated tablelands, 

 raised above the sea-level mostly in Tertiary and many in very late 

 Tertiary times. 



It is impossible to attribute the elevation of these vast flattened 

 domes to any secondary causes, such as expansion by heat of strata 

 undergoing subsidence by transmission of the isothermals. The differ- 

 ence of level is, as before explained, too great. Besides, the crust in 

 these areas must on the whole, so far from gaining in any part, suffer 

 a considerable loss of heat by the very circumstance that the heat 

 brought by the lava to the surface is lost by radiation. There would, 

 therefore, be every cause for depression of the crust were the molten 

 lakes local and independent. These areas of eruption are, on the 

 contrary, areas of elevation — not as in mountain chains by lateral 

 squeezing and an upward thrust along narrow anticlinal lines, but by 

 elevation en masse of wide portions of the earth's crust possibly 

 accompanied by fracture but without, necessarily, contortion. 



We have, therefore, in the discharge of volcanic matter prima facie 

 evidence of the existence of molten matter beneath the surface, and, 

 in the domed elevation of the surface, of a yielding substratum, fluid 

 or viscid, underlying the solid strata. Further, it follows from the 

 fact of the upheaval that the igneous rock ejected is not only replaced, 

 but that it is replaced by a quantity larger than that which is lost by 

 extravasation. This could only be effected by supplies from adjacent 

 areas of similar matter — in other words, it indicates that there must 

 be a common fluid or viscid substratum, yielding to depression in 

 some areas, and to upheaval in others, the loss in the one case being 

 counterbalanced by an addition and centralisation in others. Apart 

 from the great movements which raised the basaltic area of the Deccan, 

 Dr. Blanford* states that in the Indian Peninsula there is evidence 

 bringing down movements of elevation to the extent of 100 to 200 

 feet to so late a period as the old raised beaches (of Pleistocene age) 

 that, on the other hand, the presence of the Maldive, Laccadive, and 

 Chagos groups of atolls and coral reefs in the sea to the south-west, 

 points to slow depression ; and that there is unmistakeable proof of a 

 recent sinking of the land on the Arabian coast near the mouth of 

 the Persian Gulf. There is evidence also of recent depression in the 

 Delta of the Ganges and of the Mississippi, and probably in that of 

 the Indus. 



Though attended with more uncertainty, there is reason to believe, 

 * " Geology of India," pp. 376 and 378. 



