ROGERS, GEOLOGY OF THE CORTLANDT SERIES 27 



if all of the unstriated feldspar were considered orthoclase. The peri- 

 cline twinning is usually, of course, very faint, and much of the osten- 

 sible orthoclase is probably plagioclase twinned according to this law. 

 While this analysis shows an exceptionally low percentage of silica, 26 it 

 indicates that the identification of the orthoclase must be attended with 

 great care and that a subdivision by this criterion would be very hazard- 

 ous. At the same time, microperthite was noticed in one slide and quartz 

 in another, so that orthoclase is probably often present. 



The diorites in the hand specimen vary greatly in appearance. They 

 are generally of medium grain, although they vai " from almost felsitic 

 to an extremely coarse rock at Crugers in which the 'lornblendes are six 

 inches long. The transitions in texture are very sudden, although there 

 appears to be no concomitant change in composition. The feldspar is 

 usually white, and the rock is then distinguishable in the field, but it is 

 often a dark gray. The hornblendes are occasionally in slender laths, 

 though usually in poorly defined grains. 



The microscopic features have already been largely covered. The 

 characteristics of the feldspar have been noted; the plagioclase varies 

 from oligoclase to andesme, although it may be labradorite when the rock 

 carries hypersthene. The hornblende is usually green with strong ple- 

 ochroism: X olive green, Y brownish green, Z brownish yellow, though 

 X may occasionally become bluish green. It is rarely in laths and never 

 so when it is of the brown variety. The green variety especially often 

 carries delicate parallel inclusions, presumably of ilmenite, which re- 

 sembles somewbat the partings of diallage or enstatite. It alters to chlo- 

 rite and epidote, and in one instance secondary biotite seemed to have 

 been formed. The biotite is usually of a deep brown color, apparently 

 high in iron and showing excellent pleochroism. It may be almost want- 

 ing and may again almost replace the hornblende. In basal section, it is 

 nearly opaque. The optical angle is extremely small. It is usually asso- 

 ciated closely with the hornblende, and they appear to have been crystal- 

 lized simultaneously. Epidote, apparently original, was found in one 

 slide as figured by Williams. 27 Quartz is rare but occasionally occurs in 

 small grains. Magnetite and ilmenite, this last usually the more abun- 

 dant, occur in ordinary quantity, and pyrite and pyrrhotite are not 

 rare. Apatite, while sometimes abundant in large crystals, is usually 

 notably less than in the norites. Garnets are abundant in the locality 

 noted above. 



28 Williams (Op. cit., p. 444) states that an average of four silica determinations was 

 53.94 per cent. The particular rock analyzed by the writer is probably abnormally basic, 

 although, under the microscope, it appeared quite typical. 



27 Op. lit., p. 445. 



