438 A. A. .TULIEN — AMPHIBOLK SCHISTS OF MANHATTAN ISLAND 



toward the center of the grain, with a rather distinct curved limit, to 

 complete extinction. With continued rotation the shadow-line leaves 

 the edge of the grain and advances toward the center, there shrinks and 

 disappears. With rotation in the opposite direction, all these phenomena 

 are reversed. The concentric strain-shadows, as they may be termed, 

 plainly indicate the tension and optical disturbance produced by the 

 pinching in of such grains on all sides through pressure surviving in the 

 plicated laminae. 



One effect may also be here noted when, at a certain point during ro- 

 tation, a closed curve is presented inside of the angular outline of the 

 quartz or feldspar grain, which may simulate that of the original out- 



FiGURE '2, — Concentric Strain-shadows in Calcite Grain (X 150) observed between crossed nicols in thin- 

 section of hornblende schist. 

 Locality, Morningside avenue west, near West One hundred and eighteenth street. 



line of a clastic grain in the process of secondary enlargement. The 

 mobility and variation of this curved strain-shadow, however, in size 

 and form, render its distinction easy. 



Calcite occurs in occasional clear grains, showing the characteristic 

 cleavage, and, between crossed nicols, concentric strain-shadows, some- 

 times with well defined central shadow of a deformed hexagon (figure 2). 

 Minute nests, displaying the twinning strii)es, are occasionally recog- 

 nized in interstices of the other minerals, though never inclosed in horn- 

 blende. In slide H the cr^^stal outlines of quartz are remarkably' dis- 

 tinct in vicinity of the calcite seams, and concentric strain-shadows ver}^ 

 common and strongly marked in grains of calcite, quartz, and feldspar. 



CHEMICAL COMPOSITION OF THE HORNBLENDE SCHIST 



Through Professor Kemp, I am enabled to present chemical analyses, 

 made, at his request, b}'^ Doctor Jouet on a sample taken from many 



