On the Agency of Water in Volcanic Eruptions. 167 



fluid incandescent matter, either entirely insulated or perhaps com- 

 municating in some cases by obstructed channels, and that in these 

 subterranean molten lakes the volcanic foci originate.* 



To this view it is to be objected that the variation in depth from 

 the surface, and the existence of separate molten lakes is not com- 

 patible with the singular uniformity as a rule of the volcanic rocks 

 over the whole globe ; again, lakes would be required co- extensive with 

 large continental areas, and therefore there seems no object in the 

 limitation ; while further on this hypothesis there would seem to be 

 no available cause for the extrusion of the lava other (but applying 

 with greater force) than that assigned by Mr. Scrope, the objections 

 to which I have already named. 



•Again, the enormous outwellings of trappean and volcanic rocks 

 which took place at intervals during the Tertiary period and con- 

 tinued down to Quaternary times, afford evidence of the existence of 

 a fluid magma underlying the solid crust, co-extensive not only with 

 the existing volcanic outbursts, but also with these older eruptions, 

 and spread the volcanic phenomena over areas so large and so nume- 

 rous that it is difficult to conceive their isolation as separate and 

 independent local igneous centres. f 



In this country the great basaltic plateau of the North of Ireland 

 is 600 to 800 feet thick, and extends over an area of about 1000 square 

 miles ; those of Western Scotland are of about the same extent, 

 and it is certain that both had, before the coast denudations, a 

 much wider range. In Central France there is a still wider basaltic 

 area of yet more recent date. There are others of great extent in 

 Hungary and in Central Italy. They cover also large tracts in Asia 

 Minor, Africa, New Zealand, Australia, and America. But to confine 

 ourselves to two instances on a grand scale we may take the great 

 plateaux of Central India and of North-west America. 



In India these plateaux stretch for a distance of 500 to 600 miles 

 from north to south, and 300 to 400 miles from east to west, covering, 

 according to the reports of the Indian Survey, an enormous area of 

 not less than 200,000 square miles. % They have a general thickness 

 of from 2000 to 3000 feet, and it is estimated that the total thickness 

 of all the beds amounts to not less than 7000 feet. They are of late 

 Cretaceous or early Eocene date, and consist of a succession of beds 

 spread, no doubt, over a long period of time. 



In North America vast sheets of basaltic rocks form the high 

 plateau of Utah, while on the Pacific slopes immense regions have 



* Op. ext., p. 54. 



t See also the Address of Sir Wm. Thomson, in Section A, Brit. Assoc., 

 1876, in which the evidence regarding the physical condition of the earth is 

 reviewed. 



% " Manual of the Geology of India." 



M 3 



