94 



Cincinnati Society of Natural History. 



One large crystal, over two millimetres in length, was 

 observed in a thin section of the granite from McClenahan's 

 quarry. The crystal has been cut normal to one of the optic 

 axes, and it has an inclined extinction. 



As is well-known, allanite crystals are usually elongated in 

 the direction of their orthbdiagonal axis like epidote, but 

 usually with a more tabular development parallel to the 

 orthopinacoid. This, taken in connection with the emergence 

 of a single optic axis transverse to the direction of elongation, 



Fig. 4.— [35c] Allanite crystal broken at the pyramid. The ZOnal StrUC- 



colored than the narrow border. The pleochroism is strong 

 and b = 6 = dark brown, c=yellowish brown. The optical 

 character is negative. By the side of the large crystal is a 

 smaller one which presents the same characters, but it is cut so 

 as to show an optic axis in an excentric position. Both crystals 

 contain inclusions of magnetite, and they exhibit numerous 

 irregular cracks, which are sometimes continuous with those 

 in the surrounding epidote, and in other cases are not. The 

 large crystal is bent and broken near its center, and the 

 fracture is filled with a mineral whose gray polarization 

 colors suggest it to be calcite. On one side the epidote rim 

 continues unbroken across the fissure, but this border 

 possesses the same orientation as the allanite, and at one end 

 of the crystal the two minerals seem to grade into one another. 



would indicate that this 

 section was approximately 

 parallel to the base. This 

 also accords with the trace 

 of an orthopinacoidal twin- 

 ning which traverses the 

 section longitudinally (see 

 Fig. 4). The variably in- 

 clined extinction, however, 

 of from twelve to twenty 

 degrees indicates that the 

 section is really somewhat 

 oblique to the basal pina- 

 coid in the direction of the 



center, with epidote rim, and twinned 

 parallel to orthopinacoid. x80. 



ture is very prominent, and 

 the interior is more deeply 



