CHEMICAL CCMPOSITIUX 



439 



pounds of pulverized schist, selected from a freshly blasted ledge at West 

 One hundred and eighteenth street. I have been further indebted to 

 Doctor Jouet for special efforts toward confirmation and completion of 

 the results stated below. 





IVa. 



IV6. 



IV. 



SiOa 



42.97 



17.69 



5.09 



1.3.04 



.27 



10.00 



6.41 



.38 



1.66 



.34 



.72 



42.97 



17.39 



5.36 



42.97 



ALU, 



17.54 



FeA 



FeO 



MnO 



5.22 



13.04 



.27 



CaO 



MgO 



10.04 

 6.37 



.44 

 1.58 



.37 



10.02 

 6 39 



K2O 



39 



NagO 



1 62 



HjO 



36 



COo 



72 















98.54 



No TiO.^ was found, nor, on examination of 5 grams of the rock, any 

 evidence of Fl. The H2O was inferred from loss by ignition. The com- 

 position of the schist presents an unusually high content of alumina, iron 

 oxides, and earthy bases, whose bearing on the genesis of the rock will 

 be discussed beyond. The cause of the loss, indicated in the summing- 

 up, was not ascertained. 



DioRiTic Gneiss and Hornblende Gneiss 



These rocks are always finely granular, with slaty lamination, made 

 up, as shown on its delicately striped cross-section (plate 63), of alter- 

 nate lamellae, rarely over 0.5 millimeter in thickness, of black horn- 

 blende and of a gray-white mixture of quartz and feldspar. Biotite 

 may abound on some gUstening planes, and greenish yellow epidote, as 

 thick seams, in masses of the gneiss wliich are crumpled and distorted. 

 Specific gravity, 3.029, in a specimen from West One hundred and sixty- 

 third street and Croton aqueduct. Nine thin-sections of these gneisses, 

 from various localities on Manhattan island, were examined under the 

 microscope. 



Hornblende makes up from 30 to 50 percent of the material, partly as 

 well defined prisms, mostly as irregular scales and fibrous blades, all dis- 

 posed in the schist plane, but without the mutual parallelism noted in 

 the hornblende schist. On a cross-section, the usual cleavage traces at 

 124 degrees. Absorption scheme, r > ft > a ; r, bluish green ; ft, brown- 



