218 



University of California Publications. 



[Geology 



PETROGRAPHY OF THE SCHISTS. 



The Hornblende Schist. — This rock comes from Yum- Yum 

 Lake in the Rainy River District, Ontario, Canada. Under the 

 microscope it was the only one of the four schists whose sim- 

 plicity of structure was such as to enable a satisfactory deduc- 

 tion of heat-conductivity ratios from a knowledge of the struc- 

 ture, and of the thermal conductivities of the constituent min- 

 erals. 



The rock is entirely crystalline. The structure corresponds 

 to the allotriomorphic granular of igneous rocks. Foliated struc- 

 ture is well developed. Quartz (x) is the only mineral that 

 shows the results of dynamic metamorphism well developed, 

 and even here wavy extinction is not prevalent. 



The constituent minerals are: — hornblende, quartz (x), quartz 

 (y), orthoclase, acid plagioelase, epidote, titanite, apatite, mag- 

 netite, hematite. 



The tendency toward idiomorphism of the constituent min- 

 erals decreases in the same order as in those igneous rocks in 

 which the normal order of crystallization is followed. Thus in 

 the order of strongly developed idiomorphism come (1) apatite, 

 (2) magnetite, (3) titanite, (4) hornblende, (5) the others. 



Three slides of the hornblende schist were made, parallel 

 respectively to the bottom, side and end of the rock. 



The hornblende makes up probably a good one-third of the 

 rock as judged from the slides. The idiomorphic character is 

 not very well marked. The prism faces and the clino-pinacoid 

 are fairly well developed ; in the case of the former, this may 

 be due to the well-marked cleavage rather than to any strong 

 idiomorphic tendency. The habit is elongated parallel to c. 

 The pleochroism is very marked. The colors are: 3 — light 

 brown or yellow; b — dark brownish black or greenish black; 

 C — sea green (if very thin) to black. Absorption — b = C > 3. 



The hornblende shows a remarkable uniformity of crystallo- 

 graphic orientation. The parallelism of the longer axes could 

 be seen maeroscopically, and since the habit is elongated parallel 

 to c in every case, all c axes are parallel. The bottom and 

 side rock sections but rarely showed any prismatic cleavage. 



