THE ERUPTIVES. 351 



Magnetite or ineuaccanite is always quite abundant. It occurs in 

 small crystals or rod-like areas in the finer grained diabase to grate-like 

 forms in the ophitic diabases, and in large solid grains in the gabbro. 

 Usually it has to a greater or less degree crystal outlines. Very frequently 

 it is altered to gray leucoxene, proving that it was originally menaccanite 

 or titaniferous ma:gnetite. Some of the grate-like mingled magnetite and 

 leucoxene areas are very regular. More ordinarilv the magnetite or menac- 

 canite is but little altered. It is very often associated with secondary biotite. 



The olivine, as usual, is found in roundish granules. It is present in 

 comparatively few of the rocks, and these the fresher ones, but in one case 

 is very abiuidant. It may have been a constituent of the much altered 

 phases, but if so it has left no evidence of its presence. It is always 

 altered to a certain extent, the most usual resultant product l)eing pale 

 green material, which is taken to be serpentine, but which in some cases 

 may be chlorite. 



Plagioclase is one of the two most abundant minerals of the rock. 

 Only in a single exposure, that containing the orthorhombic i)yroxene, is 

 it subordinate in (piantity. It is always striated and is found in small lath- 

 shaped to large, broad individuals. 



The feldspars of three of the freshest diabases were separated from the 

 other constituents and analyses made. In only one of these cases was 

 there two feldspars present. No. 1 (specimen 12SS()) is from a diabase from 

 the southeast corner of Sec. 13, V: 47 N., H. 46 W., Michigan; No. 2 (speci- 

 men 9656) is from a gabbro from the center of the s(mth half of Sec. 14, T. 

 44 N., R. 4 W., Wisconsin; No. 3 A and \^ (specimen 0108) are different 

 felilspars from an oli\Tne dial)ase from the NE. \ of Sec. 13, T. 45 N., R. 

 1 W., Wisc(»nsin. The analyses are by Mi'. Thoirias M. Chatard of the 

 chemical laboratory of the United States Geological Survey. 



