No. 6. — The Intrusive and Extrusive Triassic Trap Sheets of the 

 Connecticut Valley. By William Mokris Davis and Charles 

 LivY Whittle. 



[Published by permission of the Director of the United States Geological Survey.] 



Contents. 



1. Introductory. 



2. Means of distinguishing Intrusions and Extrusions. 



3. General Features of Intrusive and Extrusive Sheets in Connecticut. 



4. Special Accounts of the more important Localities. 



5. Conclusions. 



1. — Introductory. 



The outcrops of conglomerate, sandstone, and shale in the Triassic 

 formation of tlie lower Connecticut Valley are generally inconspicuous, 

 and alone would hardly afford means of deciphering the structure of the 

 region ; but they are accompanied by ridges of strong relief, marking 

 the resistant edges of trap sheets whose close conformity to the adja- 

 cent sedimentary beds has long been recognized. It was noticed by the 

 elder Hitchcock that some of these sheets were extrusive. Manifestly 

 these are of great stratigraphic value, for after taking their places in 

 the stratified series, they constitute truly conformable members of the 

 mass, and may be used as guides to the deformatioris that the whole has 

 suffered. 1 Attention was called to their value in this respect by the 

 senior author of this essay in 1883,^ and since then something of the 

 structure of the region has been worked out ^ for the United States 

 Geological Survey by means of the dislocations of the sheets that are 

 regarded as extrusive. The field about Meriden has also been found an 



1 Chamberlin and Irving. Bull. 23, U. S. G. S., 1885, pp. 100, 101. 



'^ Amer. Journ. Science, XXIV., 1882, p. 347. Bull. Museum Comp. Zool., 

 Geol. Ser., L, 1883, p 249. 



3 Amer. Journ. Science, XXXII., 1886, p. 342. Amer. Assoc. Proc, XXXV., 

 1886, pp. 224-227. Seventh Ann. Rep. U. S. G. S., 1888. Bull. Museum Comp. 

 Zool., Geol. Ser., IL, 1889, pp. 61-87. Amer. Journ. Science, XXXVII., 1889, pp. 

 423-434. Meriden Scient. Assoc. Proc, 1889. 



VOL. XVI. — NO. 6. 



