J. F. Kemp — Extension of the Cortlandt Series. 249 



D, 8 the limestone outcrop cannot be traced because of the 

 drift, and is inferentially filled in. In C, 9, and B, 10 it is 

 however, represented by a hard, flinty, stratified rock, which 

 doubtless corresponds to ■ the slaty layers exposed at Tompkins 

 Cove. It is however very badly contorted and broken. Topo- 

 graphically, the country consists in general of high hills or 

 ridges of gneiss, surrounding a valley which is filled by the 

 eruptives. These latter aiford low hillocks and rolling ground 

 of a character peculiar to themselves. 



The massive rocks are in almost all cases, aggregates of horn- 

 blende, biotite and plagioclase, with abundant magnetite, and 

 are therefore diorites. Augite is frequent, and the common 

 accessory included minerals are as usual present. They differ 

 from the original Cortlandt in showing, so far as the writer's 

 observation has gone, no hypersthene, nor, although there are 

 individuals whose external appearance strongly resembles the 

 peridotites and pikrites of Croton Pt., have I been able to 

 identify any olivine in the slides. 



The simplest type (No. 76, from "C, 6), exhibits hornblende, 

 plagioclase and magnetite with occasional biotite. The horn- 

 blende is of both brown and green kinds. The former is in 

 large, irregular masses 5 to 20 X 50 to 200 mm ., sti;ongly pleo- 

 chroic, c, dark brown ; 5, the same ; a, yellow. The green 

 variety is feebly pleochroic, of fibrous or acicular character and 

 may be derived from the brown. The brown variety alters to 

 chlorite with secondary magnetite. The plagioclase is also in 

 large irregular masses well twinned. A sample isolated by 

 Thoulet's solution (spec. 69) afforded analysis I. 



Si0 2 



Al O 



I. 



61-12 



23-90 



II. 

 49-30 

 22-46 



Fe ,0, 





12-04 



CaO 



MgO 



K O 



._ 5-80 



. 2-58 



9-30 



• 2-14 



1-27 



2 



Na 2 



Loss on ignition 



6-78 



3-01 



•78 





100-L8 



• 100-30 



This shows it to be a nearly typical oligoclase whose soda has 

 been in part replaced by potash. The magnetite is in large 

 irregular grains and as small secondary products from the horn- 

 blende. Apatite is not infrequent. An anatysis of the rock 

 (spec: 76) gave the results in analysis II above. Three deter- 

 minations of the specific gravity gave 3 003, 3*015, 3*025. 



Starting* from this as a normal type, the simplest deviation 

 consists in the increase of biotite and relative decrease of horn- 



