Page:The Journal of geology (IA journalofgeology11893univers).pdf/441

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 contained concerning the complexity of the glacial period; a modification of the classification of geological formations of North America, incorporating the ideas of the correlation essays of the United States Geological 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 considerable. Thus twelve pages were devoted to the discussion of the Archean 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 distinctly recognized, the lower consisting of gneisses and schists, and the other of the pre-Cambrian sedimentaries and volcanics. The character, the relations, and the genesis of these groups is briefly by comprehensively 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 general 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 sedimentary 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]