224 



University of California Publications. 



[Geology 



This rock is then again unsatisfactory for making 1 numerical 

 comparisons. One would from this expect the heat conductivity- 

 ellipsoids to be nearly spheres. 



The Wrangell Mica Schist.- — This rock comes from the Stic- 

 keen River District in Alaska. Large, perfect crystals of andra- 

 dite* occur plentifully. The schistosity of the rock is due mainly 

 to the uniformity of orientation of the mica plates. The frac- 

 ture is roughly undulatory, not planar, and is good at as widely 

 different directions as those represented by 11 and 12 of the 

 figure. Almost any section perpendicular to A shows, when pol- 

 ished, beautiful mica spangles. Especially is this so of the bottom 

 section. 



Under the microscope it is seen that the micas lie with their 

 basal planes within 15° on either side of B. Hence slide 14 and 

 slide 10 (in the plane of the paper) show cleavage and pleochro- 

 ism in most sections. The cleavages are parallel enough here to 

 give the rock a uniform tint, and one which changes very decid- 

 edly as the stage is rotated; the pleochroism is very marked and 

 the absorption strong. The pleochroic colors are dark red brown 

 parallel to the cleavage and colorless to very pale grayish brown 

 perpendicular to the cleavage. The basal sections show slight 

 pleochroism. The extinction angle is uniformly very near zero. 

 The mica makes up a large part of the rock. 



Sillimanite is very plentiful in slides 14 and 12 as though 

 its distribution were roughly planar also. The needles are so 

 extremely fine that they defy detection, generally, if they occur 

 in cross section. They occur in long attenuated crystals, often 

 grouped radially. Their occurrence seems to be governed in 

 some way by the presence of the micas. The index of refraction 

 is very high, as compared to quartz by the Becke test, and the 

 dark total reflection borders generally meet and mask the color. 

 On some ciystals, however, it can be seen that the color varies 

 from colorless to a very definite pale green or bluish-green. A 

 protracted search showed transverse cracks in only three of the 

 needles. The birefringence is quite high, as even these fine 

 needles affect the interference color of quartzes lying in the same 

 vertical plane. The extension is positive. 



* Cf . analysis by A. S. Kuntze. Dana 's System of Mineralogy, p. 444. 



