454 A. A. JULIKX — AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



The little difference shown — a slight excess of iron oxides and earths 

 in the black amphibolites — only impresses the identit}^ of the two kinds. 

 The approximation of these results to those obtained with hornblende 

 schists of eruptive origin (anal3^ses XXIII to XXXI) would seem to 

 forbid discrimination on chemical grounds ; so also Emerson concludes,* 

 with reference to a black amphibolite from Washington, Massachusetts : 



" I think it probable that the rock was derived from an impure limestone, but 

 must leave its origin in doubt, because no lithological criteria can be found that 

 will distinguish amphibolites derived from lavas or tuffs and those derived from 

 impure limestones." 



In the hope of acquiring a chemical criterion by determining the com- 

 position of the amphibole derived from limestone alteration, the analysis 

 of black amphibolite of Goshen, Massachusetts, by L. G. Eakins, was se- 

 lected for study^ Professor Emerson kindly supplied me with a piece 

 from the base of the " anvil," a form produced by weathering of the rock, 

 which is described as " a quartz-hornblende rock, formed by the alteration 

 of the limestone by reaction of solutions derived from the inclosing 

 schists." t This displayed an obscure schistose structure, with irregularly 

 weaving white layers 1 to 3 millimeters thick and dark layers 6 to 9 milli- 

 meters thick ; specific gravit}^, 2.832. The dark layers consisted chiefl}^ of 

 blackish green hornblende, rather soft, with fibrous, curved, shining sur- 

 faces, in irregular grains and flakes, up to 12 millimeters long, disposed 

 somewhat parallel and in the schist plane. Under the pocket lens some 

 whitish to gray quartz was found to be interspersed through the horn- 

 blende, as well as concentrated in the white layers. The microscopic con- 

 stitution of the rock has been described in detail by Emerson.;}: 



As the slitted cross-section of the specimen possessed an area of only 

 a little over 15 square centimeters, an enlarged photograph (X 3.28) was 

 taken, and the photographic print, having an area of about 120 square 

 centimeters, was itself subjected to the method of graphic analysis de- 

 scribed beyond (see page 466) for determination of the proportion of its 

 mineral constituents. The light and dark parts were separately' cut out 

 and weighed, with further correction for the smaller portions of each in- 

 termixed with the one and the other b}^ application of Rosiwal's method 

 to a thin-section of the rock. The following were the final results so 

 obtained : 



* Op. eit., p. 30. 



t Emerson, op. cit., p. 195, and plate v, figure 1. 



X Idem, p. 191. 



I 



