PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 547 



Basalt.] 



No. 790. BASALT. ( Amygdalmdal. ) 



Portage from a small lake (in sees. 23 and 24, T. 62-1 W.) to Poplar River lake (otherwise called Pine lake); 

 the sample is from near the Poplar River lake end of the portage, and probably from N. W. % N. E. J^ sec. 25, 

 T. 62-4 W. 



Ref. Annual Report, x, page 104. 



Meg. A very tine-grained, dark-gray rock, indistinctly blotched with small 

 areas of lighter color than the main mass of the rock. 



Hie. The most noticeable feature of the section, when examined without the 

 microscope, is the presence of numerous small, white blotches, which have the 

 appearance of amygdaloidal (possibly pseudamygdaloidal) cavities filled with white 

 material. The background of the section is much darker colored than are these 

 white areas. Some of these areas have a border darker than, and passing gradually 

 into, the main mass of the section. In a few cases a white area is composed of a 

 light colored outer rim, then a dark band, and within this dark band a lighter area 

 of about the same color as the main mass of the section. These white areas 

 frequently have a brownish stain. 



Under a low power and crossed nicols the background of the section reminds 

 one of a glassy rock which has devitrified to a very finely crystalline aggregate in 

 which are some original, minute, plagioclase laths. Under a high power the 

 background of the rock is seen to consist of minute laths of plagioclase feldspar, in 

 irregular grains, and many minute and more or less rounded grains of magnetite and 

 a nearly colorless mineral; and the resemblance to a devitrified rock is not so 

 noticeable as under a low power. The nearly colorless mineral has a higher index of 

 refraction than the feldspar and also a stronger double refraction, commonly polar- 

 izing in yellows or reds of the first order, while the feldspar shows grays. This 

 mineral is perhaps aiujite. 



The lighter colored amygdaloidal areas are composed of quartz, feldspar and 

 large amounts of a granular mineral, whose index of refraction is higher than quartz 

 and whose double refraction is stronger, but apparently not so strong as that of the 

 augite-like mineral of the main mass of the rock which it resembles. This granular 

 mineral appears like epidote, but its double refraction is not marked enough for 

 epidote. In these lighter colored areas magnetite is in only small quantities. 

 The lighter and darker color of different parts of the section is due mainly to the 

 scarcity or abundance of these magnetite grains. 



One section examined. 



Age. Cabotian. u. s. o. 



No. 791. BASALT. 



West shore of Poplar River lake; probably in the E. > sec. 25, T. 62-4 W. 

 Ref. Annual Report, x, page 104. 



