378 



/. MORGAN CLEMENTS 



be very similar to the tonalite from the Rieserferner described 

 by Becke, 1 and also to the so-called grano-diorite massifs of 

 California, described by Becker, Turner, 2 and Lindgren. 3 The 

 grano-diorite of California appears from Lindgren's description 

 to correspond very closely to tonalite, though Turner uses the 

 name as synonymous with quartz-mica-diorite. 



It has not been found possible thus far to obtain analyses of 

 all these varieties. The more acid facies of the diorites seem to 

 show very clearly their gradation towards tonalites and granites, 

 and, from their content of free silica, the conclusion seems to be 

 warranted that they are rather acid. Such being the case, it was 

 deemed of more importance to study the relations of the less 

 acid dioritic facies in order to determine their relationship with 

 the basic gabbros and peridotites with which their connection 

 is not so evident as it is with the granites. To this end a com- 

 plete analysis was made by Dr. H.N. Stokes of a mica-diorite. 

 The rock analyzed consists of biotite, hornblende, plagioclase,. 

 orthoclase, and quartz, with the biotite and plagioclase as the 

 predominant characteristic constituents. 



ANALYSIS OF MICA-DIORITE BY DR. N. H. STOKES. 



Si0 2 - 

 Ti0 2 

 A1 2 3 - 

 Cr 2 3 - 

 Fe 2 3 

 FeO 

 MnO - 

 NiO 

 CaO - 



Gabbros and norites. — The gabbros and norites are holocrys- 

 talline rocks of moderately fine to coarse grain. The rocks 

 here included show a considerable variation in texture. Some 



■Op. cit. 



2 Geology of the Sierra Nevada, by H. W. Turner : 17th Ann. Rep. U. S. Geol. 

 Surv., No. 1, 1896, pp. 636-724. 



3 Granitic rocks of California, by W. Lindgren : Am. Jour. Sci., 4th series, VoL 

 III, 1897, p. 308. Here can be found reference to earlier articles on grano-diorites. 



