446 Williams — Gdbbros and Diorites of the Cortlandt Series. 



ceous quartzite,* but a careful petrographical study of it shows 

 that it is a massive rock, best to be designated as a porphyritie 

 qua/rtz-m/ica-diorite. 



Under the microscope well-formed crystals of feldspar are 

 seen imbedded in a rather coarse-grained groundmass com- 

 posed mostly of quartz and feldspar. The porphyritic crys- 

 tals possess a beautiful zonal structure and sometimes, though 

 not commonly, polysynthetic twinning striation. ]STo porphy- 

 ritic quartz occurs. 



The groundmass is a mosaic of interlocking grains unlike 

 the structure of a quartzite. It contains biotite and epidote 

 exactly like that characteristic of the mica-diorite proper, ex- 

 cept that their amount is here much less. The feldspar of the 

 groundmass is sometimes striated, sometimes not. Specific 

 gravity determinations made with the Thoulet solution show 

 all the feldspar of this rock to be plagioclase, varying between 

 2*63 and 2 - 67. This renders its separation from the quartz 

 and the quantitative determination of the latter impossible. 



This rock differs from the mica-diorite proper only in its 

 greater amount of quartz and the proportionately smaller 

 amount of biotite and epidote. It may be regarded as a vari- 

 ety of the former rock and as the most acid type of the whole 

 " Series," which is throughout essentially a plagioclastic one. 



We have now, within the limits of this and of my two for- 

 mer papers traced out the following types of basic and ultra- 

 basic rock which form members of the group called by Pro- 

 fessor Dana the "Cortlandt Series." 



Class I. Peridotite. Class IV. Diorite. 



1. Hornblende - Peridotite 1. Brown-hornblende- Diorite. 



(Cortlandtite). 2. Hornblendite. 



2. Augite-Peridotite(Pikrite). 3. Green-hornblende-Diorite. 

 Class II. Norite. 4. Mica-Hornblende-Diorite. 



1. Norite proper. Class V. Mica-Diorite. (" So- 



2. Hornblende-Norite. da-granite," " Hemidioryte," 



3. Mica- Norite. Dana.) 



4. Augite-Norite (Hyperite). 1. Mica-Diorite proper. 



5. Pyroxenite. 2. Hornblendic Mica-Diorite. 

 Class III. Gdbbro. 3. Hypersthenic Mica-Diorite. 



1. Gabbro proper. 4. Quartz-Mica-Diorite. 



2. Mica-Gabbro. 



In spite of the extent to which the subdivision of the vari- 

 ous types has been carried in the descriptions, the actual vari- 

 ety of intermediate or transitional forms has not been ade- 

 quately represented. In order to show more completely the 



* This Journal, III, xx, p. 218, Sept., 1880. 



