Crosby.] 166 [March 5, 
the stratified or sedimentary group, I will proceed to compare them > 
with the deep sea clay; first, in their textural relations, and after- 
wards chemically. 
The texture of the clay has been already described; it is nearly 
always microscopic, and might frequently, probably, be properly char- 
acterized as of almost molecular fineness. Petrosilex and felsite are 
commonly described as exceedingly intimate mixtures of the con- 
stituent minerals, — usually quartz and orthoclase, though sometimes 
orthoclase alone, for petrosilex; and feldspar, usually a triclinic 
species, with little or no free silica but often some hornblendic ma- 
terial, for felsite. In some cases the individual mineral particles are 
separable under the microscope, while in others the rock remains 
perfectly compact with the highest powers. Among those believing 
in the sedimentary origin of these rocks, the question has been raised 
as to whether mechanical or chemical forces have played the chief 
part in their formation: 7. e., as to whether they are mechanically 
formed sediment or a chemical precipitate; and some authorities 
have been led, chiefly by their almost molecular fineness, to adopt 
the latter view. In the red clay, however, we have a compromise 
between the two theories, since it consists of material which, having 
nearly reached the limit of division by mechanical means, is further 
subdivided chemically. 
The chief chemical distinction between the red’clay and the rocks 
in question arises from the dearth of alkalies in the former; but, as 
in the case of glauconite, we may look to decaying vegetation as 
a probable source of this ingredient, though this is in part unneces- 
sary, since it must usually occur that some of the pumice and other 
alkaline silicates escape entire decomposition and thus retain a por- 
tion of their alkalies. And then, again, we might, perhaps, safely 
agree with Prof. Wyville Thomson that, in consequence of the greater 
shallowness of the primeval ocean, or for some other cause, the ex- 
isting abyssal conditions failed to obtain in early geological times to 
the extent of there being suffictent carbon dioxide in the sea to effect 
the solution of the caleareous organisms and yet not enough to de- 
compose more than a small proportion of the comminuted volcanic 
débris, which we have every reason to believe was diffused far more 
profusely in ancient than in modern times. 
Dr. Jacobsen, of Kiel, and Mr. J. Y. Buchanan, chemist of the 
Challenger Expedition, have shown that the greater part of the car- 
bonic acid in sea water is not’ held in simple solution, but enters-into 
