598 



SCIENCE. 



[N. S. Vol. II. No. 44. 



serves, however, as a form of definition of the 

 terms used throughout the hook. It would 

 seem that in neglecting the use of those methods 

 of determination based on the optical phenomena 

 observed with converging polarized light the 

 author needlessly weakens the processes of 

 petrographical diagnosis. 



In his remarks upon the examination of rock 

 sections the author shows his appreciation of the 

 broad field of the science, which, as he says, is 

 not merely an attempt to discover the composi- 

 tion of a rock, but to unravel its history as well. 

 His clear understanding of the subject is also 

 shown in his discussion of the classification of 

 rocks, especially those of igneous origin. In the 

 present chaotic condition of the nomenclature of 

 rocks it will be difficult for any one who does 

 not succeed in reforming the whole system to 

 classify rocks to his own complete satisfaction 

 or to the satisfaction of any one else. In his 

 attempt at simplification Mr. Marker has shown 

 his independence to a considerable extent, while 

 following in the main the classification of igne- 

 ous rocks adopted by Rosenbusch, though un- 

 der a different terminology. Thus massive ig- 

 neous rocks are subdivided into plutonic, infru- 

 sive and volcanic, corresponding to iiefenge- 

 steine ganggesteine and vulcanischegesteine. In 

 many other ways also the author follows the 

 methods and principles of Rosenbusch. Under 

 each of the three great divisions above named 

 the rocks are arranged according to their min- 

 eralogical or chemical composition, beginning 

 with the most acid. The names used for vari- 

 eties of rocks within different families are gen- 

 erally those expressing the mineralogical char- 

 acteristics of the particular variety rather than 

 those of a geographical character which may 

 already be in common use. But in most cases 

 both names are given. The most noticeable in- 

 stance of this is in the peridotites. 



In substituting the term intrusive for that of 

 ganggesteine, and in maintaining an independ- 

 ent grouping for certain varieties of intrusive 

 rocks, the author has not improved on the pre- 

 sentation of the case as made by Rosenbusch ; 

 and his remarks in introduction of his intrusive 

 division are in the nature of an apology. 



Nor does his use of the term acid intrusives, 

 in distinction to that of porphyries and por- 



phyrites, appear to be fortunate. Diabases are 

 classed as intrusives. Under volcanic rocks no 

 distinction is made between older and younger 

 lavas, which certainly seems to be the only 

 proper method of treatment. In this respect 

 the classification follows the English usage. 

 Fragmental products of volcanic action are de- 

 scribed in connection with sedimentary rocks. 



The descriptions of the various rocks embrace 

 a general definition in mineralogical and struc- 

 tural terms, followed by an account of the con- 

 stituent minerals and of the microstructure. 

 Illustrative examples are chosen as far as pos- 

 sible ft'om occurrences in Great Britain. The 

 many references to the writings of British geolo- 

 gists and numerous others to the works of 

 foreigners add greatly to the usefulness of the 

 book for more advanced students. 



The sedimentary rocks are divided into are- 

 naceous, argillaceous, calcareous and pyroclastic 

 kinds. Under the first division the general 

 terms are defined and the characters of the de- 

 rived grains and of the authigenous constituents 

 are discussed separately. In this way the gen- 

 eral characteristics of all arenaceous rocks are 

 given rather than the specific character of any 

 one kind of rock. In the chapter on argillaceous 

 rocks the general definitions are first given, 

 then the characters of the constituent minerals, 

 followed by that of the structure. The descrip- 

 tion of illustrative occurrences serves to supply 

 the need of some definite picture of different 

 kinds of these rocks. The treatment of cal- 

 careous rocks is admirable for so condensed a 

 statement. It deals first with the source and 

 composition of these rocks ; then the structure 

 of organic fragments, followed by oolitic struc- 

 ture ; the character of the matrix and of deep- 

 sea calcareous deposits. Finally metasomatic 

 changes are described and British examples 

 cited. References to the literature of the sub- 

 ject are numerous and valuable. Pyroclastic 

 rocks are briefly treated. Deposits due to 

 chemical or to organic agencies are described in 

 a few short paragraphs. 



Under the head of Metamorphism the author 

 discusses the general principles of the subject, 

 and then describes the changes produced by 

 thermal metamorphism upon the different kinds 

 of sedimentary rocks, and upon igneous rocks 



