636 



sblENGE. 



[N. S. Vol. III. No. 69. 



Erogger is now giving to science the results of 

 his labors on the rocks of southeastern Norway 

 in a series of memoirs of which the one before 

 us is the second. Various preliminary papers 

 and the classic monograph, Die Mineralien der 

 Syenitpegmatitgdnge, have stimulated petrol ogists 

 to a keen anticipation of the magnificent contri- 

 bution which should accrue to their science by 

 the publication of Brogger's work. The first 

 two memoirs amply justify these anticipations; 

 and it is becoming apparent that the work will 

 be an epoch-making event in the history of the 

 science, and will result in the establishment, on 

 a sure basis, of the principle of magma differentia- 

 tion as one of the most important factors, if not 

 the all-controlling factor in the genesis of rook 

 types. Toward this principle, or rather toward 

 a full comprehension of its scope, petrology has 

 been groping rather vaguely for the last ten 

 years, and we now seem to have arrived at a 

 point when knowledge is beginning to crystallize 

 from the all-pervading magma of ignorance. 

 Among those prominent in contributing to the 

 modern conception of differentiation Brogger is 

 facile princeps, and it is fortunate for the science 

 of petrology that a field so rich in possibilities 

 of demonstration of the differentiation hypothe- 

 sis should have fallen to the lot of so keen and 

 masterful an investigator. 



The subject-matter of the paper may be stated 

 under the following heads: 



1. The establishment of a new family of plu- 

 tonic rocks, designated the monzonites. 



2. A discussion of the eruptive sequence near 

 Predazzo. 



3. A discussion of the mechanism of plutonic 

 eruption, involving 



4. The proof of the laccolitic character of the 

 plutonic rocks of the Christiania region. 



5. A comparison of the eruptive sequence 

 near Predazzo and Monzoni with that in the 

 Christiania region. 



6. The formulation and discussion of the law 

 of plutonic sequence, involving 



7. The discrimination between the sequence 

 of plutonic and that of volcanic rocks. 



A few words by way of summary and com- 

 ment may be of service as indicative of the 

 trend of thought in modern petrology. 



The term monzonite has been used by differ- 



ent writers in various senses as a comprehensive 

 and as a special designation for certain rocks 

 occurring in the classic environs of Predazzo 

 and Monzoni. The confusion arising from the 

 various usages of the term is historically re- 

 viewed, and it is pointed out that, however 

 various the usage, the rocks designated as mon- 

 zonites have been, with one exception, by all 

 writers, referred to the family of the Diorites, 

 or plagioclase rocks, or to the Syenites, i. e., 

 orthoclase rocks. A review of the literature 

 and of the rocks themselves leads Brogger to 

 the view that the latter are properly to be 

 classed with neither of these two families, but 

 are characterized by approximately equal oc- 

 currence of both alkali feldspar's and lime-soda 

 feldspars. This being so, he claims for them 

 recognition as a distinct family of plutonic 

 rocks intermediate between those characterized 

 by the prevalence of orthoclase (alkali feldspar) 

 and those characterized by the predominance of 

 plagioclase (lime-soda feldspar). 



After an exhaustive review of the chemical 

 characters of the monzonites and a discussion 

 of their relations to other families of rocks, he 

 formally defines them as an order of transition 

 rocks between the orthoclase and the plagio- 

 clase rocks, of true plutonic character. They 

 are of intermediate bacisity (SiO.2-49-62 per 

 cent.), with a moderate lime contents (6-7 per 

 cent.) and about the same contents of alkalies 

 in equal proportion; high in alumina (17-18 per 

 cent.) and relatively low in magnesia. Various 

 subdivisions of the monzonites are recognized, 

 such SiS pyroxene-monzonite, hornblende-monzonite, 

 etc. 



The establishment of the monzonites as a sep- 

 arate family of plutonic rocks as above defined 

 is important in the emphasis which it places 

 upon the inadequacy of the present scheme 

 of classification to accommodate all rocks, and 

 as expressive of a strong tendency among 

 petrologists to expand the nomenclature. 



The eruptive sequence near Predazzo and 

 Monzoni is formulated as follows: 



1. Oldest — Dykes and flows of basic rocks. 



2. Corresponding to the latest of these are 

 basic plutonic rocks. 



3. More acid rocks — Monzonites, represented 

 by volcanic flows of plagioclase porphyrite. 



