PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 291 



Diabase.] 



No. 265. DIABASE. (Contacting alteration.) 



Prom the upper (altered) part of No. 263. Wauswaugoning bay, where in contact with thequartzyte at the 

 coast due north by compass, from Birch island. 



Ref. Annual Report, ix, page 63; Annual Report, x, page 141; American Association for the Advance- 

 ment of Science, vol. xxx, page. 165. 



Meg. Principally reddish, rough rock, with geodic cavities containing quartz 

 and a softer, greenish substance. 



Mir. Similar to some parts of No. 263. Throughout the reddish matrix is much 

 quartz, which is in the form of graphic quartz, polarizing simultaneously over con- 

 siderable areas in the same manner as in No. 263. The quartz has been subjected to 

 a common orientation. Much of the remaining material of the slide is dark between 

 crossed nicols, being made up of finely crystalline elements. Yet it has sometimes 

 a layered structure, lying between the quartz bands as if it occupied the place of 

 some cleaved mineral, perhaps an original feldspar, whose skeleton remains in the 

 form of its original cleavage. Very rarely, also, may be seen a grain which shows 

 still a characteristic plagioclase striation. The rock in general, then, is to be con- 

 sidered a silicified eruptive. 



It contains, however, many other minerals secondary or accessory to the min- 

 erals of a basic eruptive, viz., biotite, penninite, spliene, rutile (and apparently zircon ?} 

 as detected by Prof. Lacroix. 



The biotite is sometimes closely interwoven with the chlorite, but also sometimes 

 they both fill considerable areas entirely separate. The former is also disseminated 

 widely throughout the rock. It is hardly perceptible in ordinary light, being uncol- 

 ored, but by lowering the condenser its fibrous and cleaved structure becomes 

 apparent. It is sometimes in radiating and intersecting plates which, as cut by the 

 section, show star-shaped spangles and confused knots of fine fibres. Between crossed 

 nicols it polarizes in blue, red and yellow, even in a section having no greater thick- 

 ness than 1.5 millimeters. There is evidence of this mineral changing to the next. 

 This consists of the remnants of its polarizing colors in the midst of the contrasted 

 gray colors of the chlorite. 



The chlorite has a pale, yellowish-green color in ordinary light. It is fibrous 

 and linear, with extinction parallel to the fibres, which are parallel to n f . It is the 

 principal mineral in the open geodes. It shows well the characteristic colored 

 aureoles or round dark spots. When cut parallel to the base it has hexagonal out- 

 lines, and is not distinctly fibrous. 



Spliene appears as small, yellowish and yellowish-green, isolated grains of irreg- 

 ular shapes and not abundant. Sometimes the grains are nearly colorless and have 

 the characteristic high refractive index. 



Buttle, twinned and crossed, appears in isolated, darker-yellow grains than the 

 sphene. The twinning or lamellation forms a network within the central part of 



