196 J. D. Dana— Geological Relations of the 
A. Kiyps or Rooks. 
The more prominent peculiarities in the constitution of these 
rocks, (as learned by a aid of thin slices and microscopic ex- 
amination, ) are as follow 
The feldspars are iicliy triclinic species, or soda-lime 
feldspars, though some orthoclase (potash-feldspar) is also often 
present. They are therefore distinctively what many would 
call “ plagioclase ” rocks. 
2. One or more of the minerals of the Amphibole group 
—hornblende, hypersthene, augite—are present in a large part 
of the rocks; of these, hypersthene is the most widely distrib- 
uted. Its erystals, which are sometimes quite perfect, have the 
form of the augite common in volcanic rocks except that they 
are not oblique; they were ascertained to be true hypersthene 
through optical methods by Dr. G. W. Hawes. 
3 ck mica or biotite is iitislly present, ind sometimes 
abundantly, and in some of the kinds rene wholly, or 
nearly so, the iron-bearing amphibole minera 
. Qu artz is a a ioe ingredient, ake in general is 
only sparingly pre 
5. Ch esa ae : Atinrastaitets ingredient of some of the 
common 
6. “Apatite exists in microscopic and sometimes visible crys- 
tals in all the varieties; the largest crystal observed has a 
length of half an inch and diameter of a ‘sixteenth, Magnetite 
is present in grains, and sometimes oe beds. Pyrite 
also is disseminated through most of the roc 
These crystalline rocks are commonly inate, that is, with- 
out bedding. They are everywhere jointed, and for this rea- 
son the ledges are generally piles of large and small blocks. In 
most places they undergo easy decomposition, making a gray 
or iron-red soil around; and, as the joints give access to 
water, the outer blocks in the pile have often become reduced 
to rounded and half-detached m masses. 
The rocks may be divided, for the convenience of the strati- 
graphic discussion beyond, into (1) the non-chrysolitic, and (2) 
the chrysohitic. The former include four gro ups, based on the 
iron-bearing silicate prominent in the kinds; (A) the Horn- 
blendic ; (B) the Hypersthenic ; (C) the Augitic ;. and (D) the 
Micaceous or Biotitie ; but the groups pass into one another by 
intermediate varieties. | The chrysolite-bearing kinds are 
either (E) hornblendic, or (F) au soe or AZ chiefly chrysolite ; 
but here again intermediate kinds oc 
In the following descriptions I vay confined myself to no- 
ting only the prominent distinctions so far as necessary to the 
stratigraphical discussion beyond. I take pleasure in stating 
that a detailed study of the rocks of the Cortland region has 
