182 



University of California. 



[Vol. 3. 



The maximum extinction angle measured on the albite lamellae, 

 is 9°, which indicate that it is oligoclase. It shows less decom- 

 position than the orthoclase, and no good cleavage. It makes up 

 about 20 per cent, of the rock. Some traces of poikilitie structm*e 

 were observed, the plagioelase being included in the orthoclase. 

 The muscovite occurs in flakes, averaging .58 mm. in diameter, 

 with well defined basal terminations and ragged edges. It shows 

 the usual high interference tints and strong absorption. A little 

 biotite sometimes occurs, in erystallographic continuity with the 

 colorless mica. The mica makes up about 5 per cent of the rock. 



The hornblende, when it occurs, is in small ragged crystals, 

 averaging .25 mm. in size, and of the usual characteristics. It 

 is often altered to chlorite and epidote . A few grains of magne- 

 tite are found with the hornblendes. The normal order of 

 crystallization holds, save that the microeline crystallized before 

 the quartz. 



Pegmatite. — The other type of dyke rock is a coarse grained 

 pegmatite, consisting, as seen in a hand specimen, of clear 

 quartz, cloudy feldspar, and biotite. The quartz and feldspar 

 are intimately mixed, the individual crystals of each averaging 

 1 cm. or a little larger, in size. The feldspars, to the naked eye, 

 appear to be of two varieties, one being more cloudy and show- 

 ing better cleavage than the other. The biotite recurs in large 

 flakes, arranged in all directions through the rock mass. These 

 flakes are often 5 cm. to 7 cm. in diameter. In all of this phase 

 of the rock, taken from the zone of weathering, the biotite was 

 almost completely altered to green chlorite, traces of the original 

 lustrous brown mica remaining in the centers of the largest. 

 There is often an approach to graphic structure with the quartz 

 and the feldspar. 



The type of coarse grained pegmatite passes by gradations 

 into the fine grained aplite. The grain becomes finer, muscovite 

 appears in small flakes, and with the disappearance of the biotite 

 the aplitic phase is reached. A phase illustrating very well the 

 intermediate type was found: a normal fine grained granite, with 

 both micas well developed. Also, some varieties of the dyke rocks 

 were found consisting of biotite and muscovite almost entirely, 

 the whole stained green by secondary chlorite. 



