DISCUSSION. 



Professor \V. M. Davis : How does this variation in the composition of 

 the Mesozoic traps affect Professor Dana's statement that their uniformity 

 of composition is evidence of derivation from great depth ? 



Dr. J. E. Wolff : A colorless orthorhombic pyroxene is a constituent in 

 certain Connecticut traps. 



Mr. F. L. Nason : In New Jersey, at Bull's island, a light-colored variety 

 of trap occurs which evidently contains a considerable element of pyroxene. 

 There seems to be a progressive change in the chemical composition of the 

 traps from the palisades southward. 



Professor B. K. Emerson : Is there any distinction of degree of change 

 in these rocks? The larger trap sheets in New England were more chlori- 

 tized than the smaller sheets. 



Dr. G. H. Williams : Hypersthene and olivine occurring together are 

 found to play a complementary role in volcanic rocks. In the large eruptive 

 area of the hypersthene-gabbros of Baltimore the magma is completely 

 differentiated. In certain facies, the alumina nearly disappears, and with it 

 all of the feldspar. The remaining magma crystallizes, if the proportion of 

 silica is high (54 per cent.), as a pure pyroxene rock (pyroxenite) ; if, on 

 the other hand, the proportion of silica is low (45 per cent.), as a pyroxene- 

 olivine rock (peridotite). 



(;^48) 



