300 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



On the main ridge-divide masses of conglomerate were found in a wide 

 apophysis at the southwest side of the larger stock. The magma has eaten 

 into the rock, dissolved out the cement in large amount, and has thus not 

 only thoroughly impregnated the conglomerate with granitic material but has 

 quite separated many of the larger quartzite pebbles which, still rounded, are 

 now completely enclosed in granite. The cement was evidently more soluble 

 in the magma than were the quartzite pebbles — a conclusion to be expected 

 in view of the fact that the heterogeneous cement has a lower fusion-point 

 and, in relation to the acid granite, a lower solution-point of temperature than 

 the more highly silicious quartzite. This partial absorption of the conglomerate 

 must have taken place when the magma was (because cooled down) sufficiently 

 viscous to allow of the suspension of the blocks and pebbles. At an earlier period, 

 when the cooling was less advanced, the quartzite pebbles themselves like the 

 main qruartzitic and schistose formations could have been dissolved. For 

 reasons which will be stated in chapter XXVI., the absorption of foreign 

 material in this earlier and more potent condition of the magma should not 

 be directly demonstrable on the main contacts, but it is at least possible that 

 the muscovite, which is concentrated in the endomorphic zone of the stocks, is 

 a magmatic derivative from the sericitic and feldspathic country-rock dis- 

 solved by the main body of magma in a late stage of its history. 



Quartz-diorite Apophyses. — On the 7,000-foot ridge a mile or more south- 

 east of the larger stock, the shattering of the heavily metamorphosed schists 

 is well displayed. Hundreds of irregular dikes and tongues of granular rock 

 cut the schists in all directions. From one of these a typical specimen was 

 collected and has been studied microscopically. 



Its mineralogical composition differs widely from that of the stock granite. 

 The dominant essential is andesine feldspar, near Ab 4 An,, occurring in 

 remarkably idiomorphic, twinned crystals averaging 1 mm. in diameter. Quartz, 

 which is always interstitial, is next in importance. Biotite in foils from 1 

 mm. to 4 mm. in diameter is an abundant essential. The accessories include 

 titanite, magnetite, apatite, zircon, and muscovite. The micas are often 

 regularly intergrown, with common basal plane. They show some tendency to 

 cluster in the rock and especially so where the muscovite is primary; the rock 

 is quite fresh. Not a trace of alkaline feldspar was found, though the detec- 

 tion of any such would not be difficult in this case. 



A quantitative estimate of the composition was made by the Eosiwal 

 method. A high degree of accuracy was impossible on account of the leaf- 

 shapes of the micas. Rough as it is, the estimate serves to show how widely 

 divergent the rock is from the staple granite of the stocks. The proportions 

 are as follows: — 



