REVIEWS. 



The Geology of the Igneous Eocks of Essex County, Mass. 

 Bull. 704, U.S. Geol. Surv. By Charles H. Clapp. pp. 132, 

 18 plates, 4 figs., and insert. 1921. 



rpHB writer differs from other workers in distinguishing two periods 

 -*- of batholithic intrusion, the earlier being post-Ordovician and 

 subalkaline and the later alkaline and of Devonian or Car- 

 boniferous age. 



The calc-alkali series ranges from granodiorite (68-94 per cent 

 SiO.J to gabbro-diorite (45'32 per cent SiO.J, transitional varieties 

 being developed. All these rocks show in places a gneissose or even 

 schistose structure with development of secondary minerals of 

 dynamic metamorphism ; the comagmatic characteristics are high 

 FeO and Ti02, and a predominance of Na,0 over K.^0. With regard 

 to the differentiation of this series, the author states that " it appears 

 as if this batholith is an ideal example of the differentiation in place 

 of an original basaltic magma through fractional crystallization 

 imder gravitative control." A table of specific gravities is given. 

 The variation diagram corresponds with the Pacific diagrams of 

 Harker, though the curves are more linear. 



The first rocks of the alkaline series to be discussed are the Lynn 

 volcanics, which range in composition from quartz-keratophyres to 

 andesites. They were erupted through and accumulated on a 

 weathered surface of the granodiorite mentioned in the last para- 

 graph, and are cut in jilaces by the alkaline plutouic series. 



The most acid member of the plutonic alkaline series is the 

 Quincy granite, which consists essentially of quartz, microperthite, 

 albite, and kataphorite ; hedenbergite being essential in some varieties. 

 The average composition of the microperthite is 52 per cent 

 orthoclase, though the extreme limits show 37 and 68 per cent of 

 orthoclase. Apophyses of the granite consist of granite dykes with 

 aplites and pegmatites, the latter always showing a medial portion 

 of ? /i quartz. Porphyritic varieties of the granite appear at the 

 margins. Schlieren of a slightly lower silica percentage are common 

 in the granite. 



The writer groups the pulaskites, umptekites, and hedrumites 

 under the head of Beverly syenites. The pulaskite contains a green 

 alkali-rich amphibole, like the kataphorite of the Quincy granite. 

 Microperthite and aegirite are typical of the pulaskites and 

 hedrumites, which by decrease of aegirite and increase of green 

 hornblende pass into the umptekite, and this the author believes 

 to be a hybrid between the pulaskite and gabbro-diorite. The 

 Hordmarkites (akerites of Wa.shington) are transitional rocks between 

 the syenites just dealt with and the granite. 



The author distinguishes as Squam granite the nordmarkite of 

 Washington (quartz-diorite of Staler). No new description is added 



