Ailing — Problems of Adirondack Precambrian. 63 



new occurrences showing the anorthosite cut by dikes of 

 the syenite. In the St. Regis quadrangle, just south of 

 Mountain pond a dike of quartz syenite cuts the anortho- 

 site ; the other exposure is in the Saranac Lake sheet two 

 miles west of Gabriels (Paul Smiths Station) beside the 

 state road. 



The anorthosite has been the subject of a very valuable 

 paper by Bowen. 34 From the study of the binary system 

 of solid solutions, albite-anorthite, he concluded that the 

 anorthosite, essentially a labradorite rock, was not 

 molten as such, suggesting that the anorthosite is a dif- 

 ferential phase of a gabbroic magma. He pictures a 

 huge laccolith invading and splitting the overlying Gren- 

 ville series, which differentiated into an upper layer of 

 syenite-granite and a bottom one of gabbro and an inter- 

 mediate zone of anorthosite. The suggestion of a 

 laccolith is a new conception and may furnish the expla- 

 nation of some of the obscure problems of the Algoman. 

 The writer, however, takes exception to Bowen 's view 

 that a genetic relation exists between the syenite and 

 anorthosite. The chill phases of the syenite are fre- 

 quently monzonitic to dioritic in composition. As the 

 ferromagnesian minerals are commonly pyroxenes the 

 rock as such can be called gabbroic but in no case has it 

 been the writer's experience that a "syenitic" phase of 

 the anorthosite -occurs. The writer grants the close kin- 

 ship between the anorthosite and the gabbro but ques- 

 tions a similar relation between the anorthosite and the 

 syenite, although realizing that they are nearly contem- 

 poraneous in age. 



Bowen 's suggestion that the Algoman rocks are lacco- 

 lithic rather than batholithic is a valuable one. One of 

 the problems of the Adirondack Precambrian is to 

 account for the non-discovery of the Grenville floor. If 

 the Algoman magma arose, and injected the overlying 

 Grenville as a huge laccolith, it would, perhaps, account 

 for this failure. This suggestion, however, assumes that 

 the syenite has been derived from a single mass. As 

 Cushing points out, 35 it is difficult to account for the pres- 

 ent exposures on this basis. To explain the outlying 

 syenite and granite bodies away from the anorthosite the 

 postulation of several other magmas, which may have 



34 Bowen, N. L.: The Problems of the Anorthosite, Jour. GeoL, 25, p. 

 223, 1917. 



85 Cushing, H. P. : Jour. GeoL, 25, No. 6, p. 508, 1918. 



