Trap Tuff of the Connecticut Valley. 203 





Mode and Period of the Production of the Trap Tufa. 



Such are the facts in regard to this formation. I now come 

 to the mode and period of its production. 



Two positions in relation to this subject, I think will be easy 

 to prove. The first is, that this rock must have been of contem- 

 poraneous production with the sandstone in which its beds are 

 interstratified. The very fact of their being interstratified, shows 

 that they could not have been subsequently injected. They are 

 divided into layers, if not as thin, yet often as distinct, as the 

 sandstone. Consequently, after the deposition of the sandstone 

 on which they rest, they must in some similar manner have been 

 deposited, before the layers of sandstone, now above them, were 

 formed over them. 



The second position is, that these rocks must have been the 

 joint product of igneous and aqueous agency, The stratification 

 and mechanical structure of some of the varieties clearly prove 

 the agency of water, while the vesicular and concretionary struct- 

 ure of other varieties, and the entire resemblance of others still 

 to unstratified trap, where the deposit is thick, as well as the vol- 

 canic character of the cement, even of the most decidedly me- 

 chanical layers, are equally conclusive proofs of the agency of 

 heat. Indeed there is no part of the trap formation in this val- 

 ley that presents so decidedly a volcanic aspect, as some of the 

 leathered and disintegrated hummocks of this rock. It is im- 

 possible, then, to explain the production of this rock without call- 

 "?J in the joint action of fire and water. But how and when 

 j»d this joint action take place ? I adopt, almost without modi- 

 fccation, the views advanced by Mr. Murchison, to account for the 

 °"gin of similar rocks, called by him volcanic grits, and " bed- 

 de dand contemporaneous trap," connected with the older fossil if- 

 erous rocks of Great Britain. He supposes them to have result- 

 ed from early and minor volcanic outbursts at the bottom of the 

 °? e aa> at different intervals, previous to the period when the prin- 

 ^Pal ranges of amorphous trap were protruded along the same 

 ll ] nes - These preparatory eruptions would of course throw out 

 ^undance of scoriaceous matter, finely levigated, with some 

 melted matter, which would spread over the bottom of the sea, 

 |j n d mix with the sand, gravel, and mud, there accumulated ; so 

 J? |° form the tufaceous conglomerates above described : while, 

 ™ the quantity of liquid matter was large, a portion of it might 

 001 sl °wly, and neither mix with the sand and gravel, nor come 

 , , c , 0ntact with the water, except at the surface : and there the 

 ^Uden refrigeration and commotion produced by the vapor and 

 r* s > would form scoriaceous and amygdaloidal rock. If in the 

 j»tom of the ocean this mixture of ashes, scoria, arid lava, should 

 welope the remains of animals or plants, the heat might not be 



