California, Oregon and Washington Territory. 227 
ing green when the axis ¢ is parallel to the principal plane 
of the polarizer and yellowish brown at right angles to it, of 
pens distinguished between crossed Nicols where the hypers- 
t 
more or less inclined extinction. In order to demonstrate be- 
yond all doubt by both chemical and optical tests that the 
strongly pleochroic pyroxene observed in so many thin sections 
was really orthorhombic and hypersthene, as had been sup- 
posed, it was desirable to isolate the mineral from all other con- 
stituents and examine it in a pure state. The rock selected for 
ot mercury in iodide of potassium. Under the microscope the 
Isolated pyroxene was seen to consist of about 99 per cent of 
