278 BULLETIN OF THE 
I find, fall into two divisions in the volcanic rocks: 1. Those that are 
characteristic of the rock species, and which were probably derived 
from the re-fusion of this species, that had crystallized at the depth at 
which it was prior to the eruption; 2. Those that are accidental, 
probably caught in the passage upward or during the outflow. Similar 
divisions are found, to a greater or less extent, in the sedimentary 
rocks, according as they were derived from one or more rocks, and also 
according to the preponderance of different rock fragments and miner- 
alsin them. Details of these occurrences will be given in the final pub- 
lication. 
Believing that new names should not be employed, except in cases of 
absolute necessity for filling gaps in the classification, the effort has been 
made to retain all the old names that are necessary, in their raost gen- 
eral use, and to reject all neodless ones, that can be so dealt with. 
Starting with the basic rocks, I shall pass from the glassy states to 
the most crystalline, from the least altered to the most altered, and 
from the massive to the clastic, keeping on a similar range of chemical 
composition, and tracing the various gradations step by step. I shall 
also, in like manner, trace the gradations from the basic to the more 
acidic rocks, showing the gradual changes that exist in that direction as 
well. Since, owing to the necessities of the case, both in the use of 
these observations in a thesis and in giving a post-graduate course in 
lithology in this Museum, my work was made public before it was en- 
tirely completed, it has been deemed necessary to publish this abstract, 
in advance. Several matters of detail yet remain to be worked out, 
which may modify some of the general views. All that is liable to be 
so modified must, therefore, be withheld for the present. 
Commencing with the basalts, we find a bluish-black glass, giving in 
the thin section a yellowish-brown glassy base, holding a few crystalline 
secretions. This glass passes in the same specimen into a dense black, 
globulitic base, with the same mineral secretions. We then pass on to 
specimens that contain less of the globulitic base and more and more of 
the crystalline minerals, plagioclase, a little sanidin, magnetite, olivine, 
and augite. As the rocks become more crystalline the feldspars take 
positions diverging more and more from one another, holding cuneiform 
masses of the globulitic base between them, which base, as we proceed, 
is replaced by grains of augite, olivine, and magnetite. The feldspar is 
chiefly plagioclase, and is usually in narrow, ledge-like forms. 
The olivine is mostly in rounded and broken crystals and fragments, 
showing oftentimes a blackening of its edges by the heat of the molten 
