1879.] 165 [Crosby. 
are regenerated,— glauconite being formed where the potash is de- 
ficient, and feldspar where it is abundant. 
It frequently happens that the silicious organisms always present 
in the clay predominate to such an extent as to give character to the 
deposit, which then becomes a diatomaceous or radiolarian ooze; and 
it is difficult to understand why such a deposit is not fairly represented 
by the well-known diatomaceous, or so-called infusorial, earths of 
Tertiary age, or even by the hornstones and cherts of the older form- 
ations. The radiolarian ooze has been found in the deepest parts of 
the Pacific, and nowhere at a less depth than 2250 fathoms. But 
little stress, however, can be properly laid upon the occurrence of 
diatomaceous earth and greensand in the geological formations, since 
there is no reason to doubt that these rocks were formed for the most 
part at least in comparatively shallow water. 
Among the crystalline rocks there are many kinds which, for 
aught that we can now determine, may very well have had a deep 
sea origin; but the subsequent development of crystalline characters 
has, in most cases, rendered it impracticable to trace their histories. 
In the important group of the petrosilicious rocks, however, I am 
inclined to consider that we have an exception to this statement; for 
these are at best but imperfectly crystalline, so that it may be said 
with much probability that they are texturally but little altered. In 
England and throughout Europe petrosilex and felsite, following the 
classification of Phillips, and including with the former species the 
quartz-porphyry or elvanite, are generally regarded as always purely 
and truly igneous rocks. On this side of the Atlantic, however, 
radically different views are gaining ground. Prof. Dana classes this 
group among the metamorphic rocks, and Dr. T. Sterry Hunt has 
long taught their sedimentary origin. Exotic petrosilex and felsite 
undoubtedly exist, as where a portion of a granite mass has cooled 
too rapidly for perfect crystallization and is neither vitreous nor 
crystalline in its texture, but something intermediate; or an origin- 
ally stratified petrosilex may be softened and forced out of its 
orfginal position. Of this we have the clearest possible evidence 
among the petrosilicious rocks of Kastern Massachusetts. 
But the stratification of most of the petrosilex of this and many 
other regions is too plain to be questioned, especially when their more 
general or geognostic relations point unequivocally to the same con- 
clusion, as in the vicinity of Boston, where the rocks of this group 
are clearly interstratified with stratified diorite, gneiss and quartzite. 
Considering then that the petrosilicious rocks belong normally to 
ES FS. LSE 58S _ aS SS a SS SS eee 
