April 24, 1896.] 



SCIENCE. 



637 



4. Biotite granite with contact facies of tour- 

 maline granite. 



5. Complementary dykes of camptonite and 

 neplieline-bostonite-porphyry. 



The discussion of the mechanism of plutonic 

 eruption consists chiefly in a vigorous attack 

 upon the ' assimilation hypothesis ' of Michel- 

 Levy based upon its utter failure to explain the 

 facts of the Christiania region. The assimila- 

 tion hypothesis has much in common with a 

 similar hypothesis put forward earlier by 

 Kjerulf and involves the assumption that plu- 

 tonic mcissifs have pierced the crust by a pro- 

 cess of fusion of the region invaded and conse- 

 quently of an absorption of a portion of the 

 crust, thus explaining the common abutment 

 upon these massifs of different stratigraphic 

 horizons of the region invaded. Brogger com- 

 bats this view, as it appears successfully, as 

 applied to the Christiania region, and shows 

 that the plutonic rocks have a laccolitic, and 

 not a batholitic, relation to the Silurian strata 

 which they invaded. This constitutes a very 

 important advance in our conceptions of lacco- 

 lites, the Christiania laccolite being by far the 

 most extensive now known. The assimilation 

 idea is disproved by the fact that although the 

 igneous magmas invaded Silurian limestone, 

 the analyses of the rocks show no enrichment 

 of lime near the contacts; and the fact that the 

 plutonic rocks transgress the ruptured edges of 

 the Silurian strata, with the local absence of 

 the lower members, is shown to be probably 

 due to the fact that the latter underlie the lac- 

 colite and have not been absorbed by it. While 

 the assimilation theory thus breaks down when 

 applied to the Christiania region, it is by no 

 means certain that it is not the true explanation 

 of the origin of many other more extensive 

 areas of plutonic rocks, as Brogger admits. 



The essential features of the assimilation hy- 

 pothesis were formulated by the reviewer some 

 years ago, before the publication of Michael 

 Levy's views, and urged as a satisfactory expla- 

 nation of the remarkable relations which obtain 

 between the Laurentian granites and gneisses 

 and the upper Archaean or Ontarian metamor- 

 phic rocks. These intrusive granites and gneisses 

 occupy vast tracts of the Canadian Archaean 

 plateau and there seems to be no escape from 



the view that they bear a batholitic relation to 

 the crust which they invaded from below. Por- 

 tions of the crust were absorbed, but there are 

 two possibilities as to the method of absorption 

 viz: 1. By fusion; 2. By sinking into the magma. 

 The numerous blocks of rocks scattered through 

 the granites lends much probability to the latter 

 having played a part in the process. Such 

 batholites were doubtless accompanied by lacco- 

 litic satellites. 



In his comparison of the eruptive sequence 

 in the Tyrol and Christiania regions Brogger 

 finds an essential identity to the extent that the 

 eruptive activity yielded first basic rocks, then 

 those of intermediate acidity, then acid rocks 

 and finally a reappearance of basic rocks in 

 limited amount in the form of dykes. 



The evidence bearing upon the sequence of 

 plutonic erviption, drawn from the records of 

 various well-known fields of geological research, 

 is next placed in review and leads our author to 

 the formulation of a general or normal law of se- 

 quence, which states, that plutonic rocks appear 

 in any field in the order ' basic, less basic, acid. ' 

 The sudden return to basic intrusions succeed- 

 ing the acid is ' not suflBciently constant to war- 

 rant it being made part of so general a proposi- 

 tion. This law of succession is at variance with 

 other attempts at the formulation of a general 

 law, but all such former attempts have either 

 been concerned with volcanic rocks solely, or 

 have failed to discriminate between the volcanic 

 and the plutonic. The necessity is urged of 

 investigating the succession of these two classes 

 of rocks separately. The discrimination will 

 undoubtedly lead to an elimination of much of 

 the confusion which exists in geological litera- 

 ture on these interesting questions. 



In graceful compliment to American research, 

 the volume is dedicated by its author to Prof. 

 J. P. Iddings, of the University of Chicago. 



Andrew C. Lawson. 



Berkeley, March 11, 1896. 



Electric Wiring. By Russell Robb. Mac- 

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