278 BULLETIN OF THE 



I find, fall into two divisions in the volcanic rocks : 1. Those that are 

 chai'acteristic 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- 

 als in 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 most gen- 

 eral use, and to reject all needless 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 



