Petrological Abstracts and Reviews 



ALBERT JOHANNSEN 



Harker, Alfred. Petrology for Students. Cambridge, 19 19, 5th 

 ed. Pp. 300, figs. 100. 

 A comparison of the fifth with the fourth edition of Harker 's book shows 

 the chief change to be that practically all of the American examples have been 

 "improved out." With the exception of one chapter where nepheUte- and 

 leucite-syenites are now separated from the syenites, the two editions could be 

 used in the same class, for though the pagination is different, the different para- 

 graphs can be easily located. The change of the chapter heading "Diabase" 

 to "Dolerite" will not be considered an improvement in this country. Both 

 terms are bad, in that each has two meanings; the former being used for ophitic 

 dike-rocks of the composition of gabbros as well as for Paleozoic basalts, the 

 latter, originally for coarse-grained basalts, now according to British usage for 

 the rocks we call diabase. Harker still clings to the classification of diorite as a 

 hornblende-bearing rock (p. 63) while gabbros are defined as "characterized by 

 pyroxenes in place of hornblende " although he recognizes the modern tendency 

 in classifications when he says: "The distinction between the hornblende- and 

 augite-bearing types is rather an artificial one. It was established before the 

 strong tendency of aUgite to pass over into hornblende was thoroughly appreci- 

 ated." Also, "The family as so defined cannot be regarded as a natural one." 

 The feldspar of diorite is given as "andesine or labradorite or exceptionally a 

 more basic variety," while in gabbros it is given as "labradorite, with exception- 

 ally one more acid and occasionally orthoclase." 



HiRSCHWALD, J. Leitsdtze fur die praktische Beurteilung, zweck- 

 mdssige A uswahl und Bearheitung natiirlicher Bausteine. B eriin, 

 1915. Pp. 36, figs. 18. 

 This book was primarily written for stone-workers and aims at giving the 

 most important points needed in the selection and judging of building-stone. 

 There is a short classification of rocks giving the desirable and undesirable 

 characteristics of each, a chapter on the examination of stone quarries with 

 examples of written forms for describing quarries, and chapters on the essen- 

 tials of stone-testing, advantageous modes of working and using certain rocks 

 with reference to schistosity, cleavage, etc., various uses of different kinds of 

 stone, etc. 



