REVIEWS. 857 



sea exploring expeditions, as to sedimentation far out from land ; the 

 results of recent biological investigation touching the supply of lime 

 carbonate and silica from which animals and plants secure materials 

 for their shells ; a more explicit statement than the earlier edition con- 

 tained concerning the complexity of the glacial period ; a modification 

 of the classification of geological formations of North America, incor- 

 porating the ideas of the correlation essays of the United States Geo- 

 logical Survey, etc., etc. The additions and changes concerning these 

 topics fairly represent the character of the alterations to be found 

 throughout the volume. These new touches are sufficiently numerous 

 and suggestive to make the volume valuable, even to those already in 

 possession of the earlier edition. 



At a number of points the changes have been much more consid- 

 erable. Thus twelve pages were devoted to the discussion of the Arch- 

 ean in the old edition, while thirty-seven pages are given to the pre- 

 Cambrian in the new. The general character of the changes at this 

 point were foreshadowed in an article by Sir Archibald in the first 

 number of this journal. Two groups of pre-Cambrian rocks are dis- 

 tinctly recognized, the lower consisting of gneisses and schists, and the 

 other of the pre-Cambrian sedimentaries and volcanics. The charac- 

 ter, the relations, and the genesis of these groups is briefly but com- 

 prehensively set forth. Concerning the first group the conclusion 

 reached, as expressed in the author's own words, is as follows : 



"These rocks are in the main various forms of original eruptive 

 material, ranging from highly acid to highly basic; they form in gen- 

 eral a complex mass belonging to successive periods of extrusion ; some 

 of their coarse structures are probably due to a process of segregation in 

 still fluid or mobile, probably molten, material consolidating below the 

 surface ; their granulitized and schistose characters, and their folded 

 and crumpled structures point to subsequent intense crushing and 

 deformation ; their apparent alternation with limestone and other rocks, 

 which are probably of sedimentary origin, are deceptive, indicating no 

 real continuity of formation, but pointing to the intrusive nature of the 

 gneiss." 



Concerning the second group of pre-Cambrian rocks, the sediment- 

 ary and volcanic series, Sir. Archibald [takes the same position as in 

 the article already referred to 1 and essentially the same position as that 

 of Prof. Van Hise, already set forth in this journal 2 and elsewhere. 3 



'This Journal, Vol. I., p. 1. 



2 Vol. I., No. 2, p. 123. 



3 Bulletin 86, United States Geological Survey. 



