158 J/. IV. TURNER 



1730 may be stated as follows: oligoclase> quartz ;> micro- 

 cline> biotite. 



Quartz-diorite-aplite . — In the bed of Bear River, Big Trees 

 quadrangle, there are small white dikes from two to ten centi- 

 meters or more in width, occupying straight fissures in gneiss 

 and quartz-diorite. The dikes have an aplitic texture and are 

 much more acid than the quartz-diorite. It may be assumed 

 that they bear a genetic relation to the diorite, similar to that 

 existing between the potash-aplites hereafter described and the 

 granodiorite and quartz-monzonite. In the table of analyses 

 with the soda-granites there is given the chemical composition 

 of one of these dikes (No. 1490) as well as that of the quartz- 

 diorite (No. 1495) in which they occur. No. 1490 1 is practi- 

 cally an aplite, the feldspar, however, being probably chiefly 

 andesine. The enclosing quartz-diorite is quite basic and we thus 

 have a suggestion that the composition of aplitic dikes is deter- 

 mined by the composition of the granitic rock in which they occur. 

 Rosenbusch 2 refers to tonalite-aplite and diorite-aplite and 

 the dikes above described might be designated tonalite or 

 quartz-diorite-aplite, following Rosenbusch. It should be noted, 

 however, that by some authors the term aplite is restricted to 

 granites composed chiefly of quartz and alkali -feldspar. If, 

 however, dikes occur in various magmas which, while varying in 

 composition, show a direct genetic relation to these magmas, 

 some group term for such dikes is desirable. 



The potash-aplites and pegmatites of the granodiorite series. — At a 

 great number of points in the Sierra Nevada, there are dikes of a 

 white rock from a few inches to a few feet in width. In the 

 granodiorite and quartz-diorite these dikes are usually medium- 

 grained with only occasional dark constituents. They grade 

 over into pegmatite. The pegmatitic facies will, however, be 

 treated in a later paragraph. This aplitic granite is composed 

 of quartz> potash-feldspar > sOcla — lime-feldspar (oligoclase)> 

 biotite > magnetite > apatite. 



1 Seventeenth Ann. Rep. U. S. Geol. Surv., Part I, p. 704. 



2 Mikroskopische Physiographic der massigen Gesteine, 1896, p. 464. 



