HISTORY OF CRYSTALLINE ROCKS. 21 
concludes that “in the ferro-calciferous layer from the Primary to the Tertiary period. . . . 
there was a sensible diminution of silica and potash, and a notable augmentation of soda 
and lime.” Of these changes, “the diminution of silica and potash in the modern rocks, 
both of the acidic and basic groups,” was by Durocher explained by supposing that while 
these imaginary igneous layers remain distinct from each other, there is, nevertheless, 
in each a partial separation of these elements, by gravity, resulting in an accumulation of 
silica and potash in their upper portions, and of lime in their lower portions. The 
augmentation in the proportion of soda was by him referred to a special and independent 
cause, the supposed “intervention of sea-water in the formation of igneous products 
during the latter geological periods,” which, as he writes, would explam “the considerable 
increase of soda in the more modern of the igneous rocks, whether they be derived from 
the acidic or the basic layer.” 
§ 34. While Durocher included in the category of eruptive rocks certain masses, 
such as those of magnetite, serpentine, and various amphibolic rocks, for which an igneous 
origin is not admissible (so that some of his data may be questioned), the correctness of 
his important generalizations, which suggest a vast geogenic problem, cannot be con- 
tested. As regards his proposed explanation, it is easy to conceive that a separation by 
specific gravity might possibly cause such variations, alike in the acidic and the basic 
layer, that the ejections in the course of ages from successively lower portions of each of 
these would show the gradual diminution observed in the proportions of silica and po- 
tash, as well as the augmentation of lime. To this ingenious explanation, however, it is 
to be objected that it is based upon the unproved and, in the opinion of many modern 
philosophers, the untenable hypothesis of a molten substratum, and, moreover, one divided 
into two distinct zones. The whole of the phenomena in question, moreover, admit of a 
simpler and, it is believed, a more probable explanation, by the crenitic hypothesis. This, 
as we have seen, supposes a constant and progressive differentiation of an original basic 
plutonic mass through the action of water, which removes therefrom, in the elements of 
orthoclase and quartz—the chief constituents of granitic rocks,—preponderant proportions 
of silica and potash : an action which would result at last in the partial exhaustion of the 
lixiviated portion of the basic rock, which, with the diminution of the amount of available 
silica and potash, would finally yield to the solvent action of the waters only the elements 
of the more basic feldspars. As a result of this continued process, the crenitic products 
themselves will naturally show a diminution in the proportions of silica and potash, by 
reason of the progressive exhaustion of the source of these, while this residual portion of 
basic rock will not only exhibit a reduction in the proportions of silica and potash, but a 
relative increase in the proportion of lime. Moreover, the sodium and magnesium-chlorids 
which, from the results of subaerial decay, find their way into the surface-waters, which 
subsequently pass downwards in the process of lixiviation, may, by double exchange, 
effect the displacement of potash and the fixation of soda and magnesia in the basic mass, 
as explained farther on. 
§ 35. This hypothesis thus explains at the same time the origin of the highly silicic 
and potassic rocks, represented by the granites, and the conversion of the original plutonic 
stratum into a more and more basic material, progressively richer in alumina, soda, lime, 
and magnesia. It moreover requires that the long-continued lixiviation of a given area 
of plutonic rock should at length reach a point at which water could no longer remove 
