468 Reviews — Clarence King — Survey of Fortieth Parallel. 



Archaean time and proceeding without break through the entire series 

 to the Quaternary. The first chapter contains the leading geographic 

 features of the area, which, however, are more fully treated of in the 

 second volume. The second to the sixth chapters contain a detailed 

 account of the Archsean, Palaeozoic, Mesozoic, and Cainozoic rocks, 

 the seventh chapter comprising the history of the Tertiary volcanic 

 rocks. 



The characters of the Archsean rocks are treated of under the 

 different localities where they are exposed ; these rocks are con- 

 sidered to attain a maximum thickness of 50,000 feet, and comprise 

 the Laurentian — consisting of red orthoclase granite, mica gneisses 

 and schists, with deposits ■ of ilmenite and graphite, and the 

 Huronian — composed of plagioclase-horn blende granites, diorite- 

 gneisses, argillites, limestones and quartzites. Between the rocks 

 referred to Laurentian and Huronian ages, there is a characteristic 

 difference in the intensity of the metamorphism which has taken 

 place and also in the obliteration of original structure and in the 

 increase in thickness, when considered in relation to depth. The 

 lowest Laurentian aplitic granitoid bodies of the Laramie Hills are 

 the heaviest beds and the most changed from their original sedi- 

 mentary condition ; the higher Huronian group of gneisses, 

 quartzites, conglomerates, dolomites and argillites are at once the 

 most thinly-bedded and the least metamorphosed. 



The granites are divided into four groups : — 1. With muscovite ; 

 2. With little or no piagioclase and biotite ; or 8. With biotite and 

 hornblende ; and a 4th type more complex than the others, con- 

 taining a high per-centage of biotite, with an equal proportion of 

 hornblende, the piagioclase often equal, or sometimes exceeding in 

 quantity the orthoclase and titanite ; the muscovite type being the 

 oldest, and the dioritoid variety being the youngest. These granites 

 and the crystalline schists are pre-Cambrian. 



The section (p. 112) on the genesis of granite and crystalline 

 schists contains some points which of late years have occupied the 

 attention of geologists, and embodies the author's views as to their 

 comparative origin. 



In alluding to Dr. Sterry Hunt's theory, that early magnesian 

 silicates are chemical precipitates from the acid ocean of their period, 

 Mr. King sees no reason to seek for a different origin for the 

 magnesian silicates from that of the commoner aluminous minerals, 

 and so far as the gneisses and crystalline schists are concerned, he 

 is led to give in a complete adhesion to the hypothesis of diagenesis 

 for the anhydrous silicates, and of subsequent pseudomorphism for 

 the hydrous magnesian rocks. 



'•' In the crystalline schists and gneisses are found identically the 

 same minerals which characterize the granites. The characteristic 

 features of the schists are, the parallel-bedded arrangement, the 

 strict retention of chemical materials in their original zones, and the 

 intercalation of beds made of simple materials, like quartzites and 

 limestones. The sole difference seems to be that granite is often 

 demonstrably a plastic intrusion, and possesses no parallel arrange- 



