Bibliography. 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- 

 canic injections are known to produce, namely, vesicular scorise, 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 

 its 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. 



* Our author, however, justly remarks, " that new classifications may be pro- 

 posed by the scientific men now engaged in collecting facts, but that none of those 

 local appellations so frequently put forth by geological writers ought to be univer- 

 sally adopted, until opportunities for a general discussion take place, which will 

 ere long be effected by the union of the transatlantic geological societies and those 

 of this country." 



