relations of the Metamorphic Rocks. 215 
with the progress of geological science, a new problem is pre- 
sented to his investigation. While paleontology has shown that 
the fossils of each formation furnish a guide to its age and stra- 
tigraphical position, it has been found that sedimentary strata of 
all ages up to the Tertiary, inclusive, may undergo such changes 
as to obliterate the direct evidences of organic life; and to give 
to the sediments the mineralogical characters once assigned to 
primitive rocks. The question here arises, whether, in the ab- 
sence of organic remains, or of stratigraphical evidence, there 
exists any means of determining, even approximately, the geolog- 
ical age of a given series of crystalline stratified rocks;—in other 
words, whether the chemical conditions, which have presided 
over the formation of sedimentary rocks, have so far varied, in 
the course of ages, as to impress upon these rocks marked chem- 
ical and mineralogical differences. In the case of unaltered sedi- 
ments, it would be difficult to arrive at any solution of this ques- 
tion without greatly multiplied analyses; but in the same rocks, 
when altered, the crystalline minerals whieh are formed, bei 
constitution of the sediments, may, perhaps, to a certain extent 
ecome to the geologist what organic remains are in the unal- 
tered rocks, a guide to the geological age and succession. 
It was while engaged in the investigation of metamorphic 
Tocks of various ages in North America, that this problem sug- 
gested itself, and I have endeavored from chemical considera- 
tons, conjoined with multiplied observations, to attempt its so- 
lution. In this Journal for 1858, and in the Quarterly Journal 
of the Geological Society of London for 1859, (p. 488), will be found 
the germ of the ideas on this subject which I shall endeavor 
to explain in the present paper. : 
It cannot be doubted that, in the earlier periods of the world’s 
history, chemical forces of certain kinds were much more active 
than at the present day. Thus, the decomposition of earthy and 
alkaline silicates, under the combined influence of water and 
carbonic acid, would be greater when this acid gas was more 
abundant in the atmosphere, and the temperature probably 
leates of alumina, combined with silicates of potash, soda and 
lime, these latter bases are removed, together with a portion of 
Siliea; and there remains, as the final result of the process, a hy- 
drous silicate of alumina, which constitutes kaolin, orclay. This 
