136 G. H. Williams—WNorites of the “ Cortlandt Series.” 
and apatite are always present, though in varying quantities, 
and garnet is not uncommon, especially around the edge of the 
area. 
The predominating rock-type within the Cortlandt area is 
particularly characterized by the presence of hypersthene. 
Throughout nearly the entire township east of the New York 
Central and Hudson River Railroad and also, to a considerable 
extent west of this, rocks of this class prevail; and still ex- 
amples of the pure norite type, composed wholly of plagioclase 
and hypersthene, appear to be exceptional. In nearly all speci- 
mens biotite, hornblende and augite, either singly or together, 
accompany the hypersthene in every conceivable proportion 
and thus form gradual transitions into mica-diorite, hornblende- 
diorite and gabbro. These rocks are developed in considerable 
purity in the western portion of the township; and yet, through- 
out the entire Cortlandt region, intermediate and transitional 
forms are everywhere more abundant than well defined ex- 
tremes. The naming and classification of particular specimens 
becomes a matter of great difficulty; and when it is remem- 
bered that to the mineralogical variety a great structural diver- 
sity is added, some idea may be formed of the number of 
different types which may be collected within this very limited 
district. 
The constant occurrence of such transitional forms and the 
want of any regularity in the distribution of the pure types, 
makes it impossible to regard these rocks as anything but local 
modifications or special facies of one and the same mass. . How- 
“ever great their mineralogical variety may be, they together 
form but a single geological unit. 
As already stated there seems to be a peripheral distribution 
of those rock-types which, like diorite and gabbro, differ most 
widely from the prevailing norite. A possible connection be- 
tween this fact and the proximity to these rocks of the mica- 
schists and limestones which surround or are enclosed in the 
massive area, the writer hopes to discuss, in connection with 
some chemical analyses, in a later paper. 
At present only a petrographical description of the massive 
rocks themselves will be attempted. Tor this purpose the pure 
types will be treated in succession and about each will be 
grouped such varietal or transitional forms as are produced by 
the various admixtures of accessory constituents. Hach class 
will be designated according to its prevailing non-feldspathic 
mineral as follows: 
Class II, hypersthene, Nortre. 
“III, augite, GABBRO. 
“ IV, hornblende, Drorrrs. 
1 Ve biotite, Mica-Driorire. 
