AMERICAN MINERALS 149 



posed of Pyroxene, more or less confusedly crystal- 

 lised, and varying considerably in tbe size of its crys- 

 taline particles. It is probable tbat tbere is an in- 

 termixture of a small quantity of Feldspar, from the 

 difference of colour, which the perceptible and 

 imperceptible particles exhibit when in a state of de- 

 composition. From the general character of tbese 

 veins, tbeir total dissimilarity with all rocks of the 

 class to which the granite belongs, from their being 

 composed of Pyroxene and of the trkinitaire form 

 so common in almost all lavas, I think in the present 

 state of our knowledge, (as to the origin of rocks,) 

 that we are in some measure authorized in consider- 

 ing them to be of Volcanic, rather than of Neptunian 

 origin." 



Descriptions of Univalve shells of the United States. 

 By Thomas Say. 



The terrestrial and fluviatile shells which form 

 the subject of the following pages, were chiefly ob- 

 tained on the late expedition to the Rocky Moun- 

 tains, under the command of Major Stephen H. 

 Long. They are now deposited in the Philadelphia 

 Museum, and constitute, in the collection of that in- 

 stitution, a distinct arrangement. 



A few descriptions are added to this essay, of 

 shells discovered in East Florida, Alabama, Penn- 

 sylvania, and New- York. 



