THE ORIGIN OF IGNEOUS ROCKS. 
103 
rocks derived from the same magma the differences in the 
mineralogical characters depend less upon their elementary 
composition than on conditions of pressure, temperature, 
and, in general, on the circumstances of their cooling—that 
is to say, on conditions of an external character rather than 
on those of an internal character.” * 
The zone of contact between these two magmas produces 
intermediate rocks or “ roches liybrides .” 
The permanence of the separation of the two magmas is 
due to the difference in their specific gravities. There is, 
therefore, a fluid zone beneath the solid crust, the upper 
part of which is silicious, the lower basic. It would scarcely 
seem necessary for Durocher to consider this continuous 
body of molten magma, the middle of which is inter¬ 
mediate in character between the top and bottom portions, 
as two bodies of magma, except that he conceived the inter¬ 
mediate rocks to constitute a very inferior part of all igne¬ 
ous rocks. His conception more closely resembles Von 
Waltershausen’s than Bunsen’s; but he realizes that recent 
eruptive rocks are both, silicious and basic, and correspond 
to ancient ones in composition, and consequently that his 
zone of magma must be the same now as in early geological 
ages, and capable of erupting silicious magmas as well as 
basic ones. 
In order to explain certain minor variations in igneous 
rocks Durocher introduces the process of liquation , and says 
that “ the magmas which have produced igneous rocks are 
comparable to baths containing several metals in a state of 
fusion, and which upon congealing separate into different 
alloys, according to the circumstances of their solidification, 
* Op. cit., p. 220: 
“ Pour les roches derivant d’un meme magma, les differences dans les 
caracteres mineralogiques tiennent moins a leur composition elementaire 
qu’ a des conditions de pression, de temperature, et, en general, aux cir- 
constances de leur refroidissement, c’est-a-dire a des conditions d’un ordre 
externe plutot que d’un ordre interne 
