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the one beneath the other, according to their specific gravities. The : 


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upper or outer layer, which he termed the acid or siliceous magma, 
contains an excess of silica, and has a mean density of 2°65. ‘The 
lower or inner layer, which he called the basic magma, has from six 
to eight times more of the earthy bases and iron oxides, with a mean — 
density of 2°96. ‘To the former he assigned the early plutonie rocks, 
granite, felsite, &c., with the more recent trachytes ; to the latter he 
relegated all the heavy lavas, basalts, diorites, &c. The ratio of | 
silica is 7 in the acid magma to 5 in the basic. Though the propor- 
tion of this acid or of the earthy and metallic bases cannot be 
regarded as any certain evidence of the geological date of rocks, nor of — 
their probable depth of origin, it is nevertheless a fact that (with 
many important exceptions) the eruptive rocks of the older geological 
periods are very generally super-silicated and of lower specific 
gravity, while those of later time are very frequently poor in silica, 
but rich in the earthy bases and in iron and manganese, with a 
consequent higher specific gravity. The latter, according to 
Durocher, have been forced up from a lower zone through the lighter 
siliceous crust. ‘The sequence of volcanic rocks as first announced 
by Richthofen, has an interesting connection with this speculation. 
The main mass of the earth’s crust is composed of a few pre- 
dominant compounds. Of these in every respect the most abundant 
and important is Silicon dioxide or Silica (Kieselerde) Si O,. It forms 
more than one half of the known crust, seeing that it enters as a 
main ingredient into the composition of most crystalline and frag- 
mental rocks. It oceurs in the free state as the abundant rock- 
forming mineral quartz. Being one of the acid-forming oxides 
(H,SiO,, Silicic acid, Kieselsiure) it forms combinations with 
alkaline, earthy, and metallic bases which appear as the prolific and 
uuiversally diffused family of the silicates. Moreover it is present in 
solution in terrestrial and oceanic waters, from which it is deposited 
in pores and fissures of rocks. It is likewise secreted from these 
waters by abundantly diffused species of plants and animals (diatoms, 
radiolarians, &c.) It has been largely effective in replacing the 
organic textures of former organisms, and thus preserving them as 
fossils. 
Alumina or Aluminium oxide (Thonerde), Al,O,, occurs sparingly 
native as Corundum, which, however, according to F, A. Genth, was 
the original condition of many now abundant complex aluminous 
minerals and rocks. ‘The most common condition of aluminium is 
in union with silica. In this form it constitutes the basis of the vast 
family of the aluminous silicates, of which so large a portion of the 
erystalline and fragmental rocks consists. Exposed to the at- 
mosphere, these silicates lose some of their more soluble ingredients, 
and the remainder forms an earth or clay consisting chiefly of 
silicate of aluminium, 
Carbon in the various kinds of coal takes rank as an important 
rock-forming element. But its most universal condition is in carbon 

