478 A. A. JTLIEN AMPHIBOLE SCHISTS OF :SIANHATTAN ISLAND 



tlie latter, larger crystals of amphibole. long prisms, blades, and needles 

 are not at all uncommon, recognized as due to secondary development of 

 black hornblende, actinolite, and tremolite ; these have been alread}" de- 

 scribed (page 444). Distinct from them there remain, in part of the horn- 

 blende schist and dioritic gneiss, obscure dark blotches or flattened flakes 

 of rhombic, rectangular, or ovoid outline, 1 or 2 centimeters in length- 

 Although under close inspection they turn out to be aggregates of ordi- 

 nary grains of hornblende, they undoubtedly represent in some cases 

 vaguel}^ defined remnants of ancient phenocrysts. 



In a search for survival of these forms the heavier beds of hornblende 

 schist invited special attention. Small specimens from beds on the east 

 side of the island have occasionalh^ presented a ver}^ coarse texture, as 

 if made up of columnar grains l3'ing in all positions, but always con- 

 sisting of hornblende. Similar coarse amphibolites were observed at 

 the West Fifty-ninth Street locality, but always consisting of hydrated 

 or serpentinized tremolite. The large bed at West One hundred and 

 thirty-fifth street w^as found so thoroughly sheared, crumpled, thinly 

 laminated, and epidotized as to forbid any hope of detection of original 

 texture. Possible examples of porphyroidal texture, however, were 

 noticed at the West One hundred and nineteenth street. One hundred 

 and thirty- eighth street, and One hundred and sixty-fifth street localities. 



In the largest bed of hornblende rock now left on the island, that on 

 Spuyten Duy vil creek, the mass offers little evidence of pressure and dis- 

 tortion aside from some folding. Apparently in consequence of this, its 

 material, a fine-grained dioritic gneiss, is quite uniforml}^ mottled with 

 dull black grains, 0.5 to 3.0 centimeters in diameter, at intervals of 1 or 2 

 centimeters. Their contours, though in part angular, are generally 

 irregular, rounded, or ovate, resulting in a miniature augen-structure- 

 Each grain is a composite aggregate of hornblende rods lying in various 

 positions. These grains therefore represent altered phenocrysts of a 

 ferromagnesian mineral in an ancient porphyry, with forms partially 

 distorted by shearing. 



STRUCTURAL EVIDENCES OF IGNEOUS ORIGIN 



The harmonization of such opposite qualities, extremes of plasticity 

 and rigidit}'' in the same rock, shown by features of structure already 

 described (page 429), yet calls for discussion. 



In regard to the extraordinary plication of the hornblende schist, Dana 

 offers the following explanation : * 



"The presence of hornblende or hornblendic schist appears to have often deter- 

 mined a crowd of subordinate flexures and contortions in the beds, and a loss of 



*Am. Jour. Sci., vol. xxi, 1881, p. 429. 



