. pitllbonty, 183 
» | This able report exhibits, as we might expect from the high charac- 
ter of its author, abundant proof of laborious, careful and skillful in- 
vestigation, and, both in its scientific and practical bearings, forms al- 
together a valuable document. In determining the geological age of 
rocks, Dr. Jackson gives a preference to “superposition of strata and 
the mineralogical composition” over “ zoological and botanical char- 
acteristics,” which however he allows to be “of great value.” He 
prefers also the Wernerian division of transition rocks to the “ names 
Cambrian and Silurian, proposed for certain groups in England,” 
which he thinks ‘“ will never be regarded in this country as appropri- 
ate terms for our rocks.’ 
While we agree with Dr. Jackson that a successful substitute for 
the transition division has never yet been made, we are inclined to 
think that Mr. Conrad, Mr. Vanuxem, and their associates, have so far 
identified our great western fossiliferous formations with the Silurian 
and Cambrian of Mr. Murchison, that his names will be found to be 
convenient appellatives for vast regions of our country, subordinate to 
the more extensive class of transition.* 
Dr. Jackson has justly magnified the importance of the fusion of 
chalk under immense pressure, by Sir James Hall, and its conversion 
into crystallized limestone without the loss of its carbonic acid, and he 
has found in the intrusive greenstone and other trap dykes among the 
sandstone strata of Maine and Nova Scotia, the same results that vol- 
a injections are known to produce, namely, vesicular scori, in- 
creased hardness, and moreover in particular places the separation of 
metallic copper, evidently by fusion and reduction from its ores. 
We wish we could feel satisfied with the author’s ingenious sugges- 
tion that “ gneiss is the mere crust of rapidly cooling granite.” How 
can this be reconciled with the immense thickness as well as extent of 
ils strata in the mountains of New England and in other parts of the 
World, and with the extremely limited and slow conduction of heat 
through masses of rock? ‘The cooling, it is true, would begin on the 
surface, and would travel inward, through no matter how long an ex- 
tent of time; but how would the laminar arrangements arise, thousands 
of feet from the surface, any more than in the subjacent granite nu- 
cleus or substratum? Granites, it appears, differ very much from each 
other in fusibility, and minerals still more infusible are produced by 
Segregation both in granites and lavas. 
