Geology and Mineralogy. 155 



47 plates. London, 1888 (Dulau & Co., Soho Square). — The nu- 

 merous active workers in English petrography, among whom the 

 author of the present work is included, have shown that a re- 

 markably wide range of igneous rocks is included in the con- 

 fines of Great Britain. To give a complete and systematic ac- 

 count of these, within the compass of a single volume, is a seri- 

 ous task, but one which the author has accomplished in an admir- 

 able manner. A minute personal knowledge of the rocks them- 

 selves, and wide reading of the literature of the subject abroad, 

 has equipped him well for the work. The opening chapters are 

 devoted to a general statement as to the forms taken by the 

 constituents of igneous rocks, and to a discussion of the chemical 

 and physical characters of the rocks. The author wisely regards 

 rocks, not so much from a narrow mineralogical point of view, 

 with reference only to the minerals of which they are composed, 

 but more broadly with regard to their general chemical composi- 

 tion. He also smows independence in cutting loose from the age 

 criterion as a fundamental basis for classification and nomencla- 

 ture. He insists strongly upon the distinction between the basic 

 and acidic members of the rock series. In this connection he 

 emphasizes the effect of crystallization on an igneous magma to 

 increase the relative amount of silica and alkalies, and further in 

 the alkalies to increase the potash relatively to the soda. He 

 quotes here a series of analyses, from widely different sources, 

 bringing out these facts and refers to the work of Rosenbusch as 

 showing that they are confirmed by microscopic examination. 



The classification adopted divides the rocks into a series of groups 

 passing from the basic to the acidic, according to the chief min- 

 eral constituents present; further, under each group the distinc- 

 tion in structure between "granitic" and " tr achy tic texture" 

 forms a basis of further subdivision. 



The first group includes the so-called ultra-basic rocks, that is, 

 those made up of olivine alone, or with some form of pyroxene or 

 amphibole without feldspar. The basic group follows, the rocks 

 characterized by plagioclase with olivine, pyroxene, amphibole 

 and biotite. This passes through the rocks of intermediate com- 

 position to the third group characterized by the presence of or- 

 thoclase with the various other minerals named. The rocks con- 

 taining nephelite and leucite follow and then certain forms 

 not easily included in the preceding divisions. The glassy and 

 fragmental rocks close the series. Under each group a general 

 summary is given of the constituent minerals and of the rocks as 

 they appear elsewhere ; in this portion of the work the author 

 shows a full knowledge of the extended literature of the subject. 

 Then follows a somewhat minute account of the British rocks 

 falling into the place, based upon the exhaustive studies of Judd, 

 Allport, Rutley, Worth, Teall and others. Of the various rock 

 types falling into the groups mentioned, the majority are fully 

 represented and have been thoroughly studied ; certain kinds, 

 however, for example those of the nephelite, leucite series, the 



