No. 6. — The Intrusive and Extrusive Triassic Trap Sheets of the 
Connecticut Valley. By WittiamM Morris Davis and CHARLES 
Livy WHITTLE. 
[Published by permission of the Director of the United States Geological Survey.] 
ConTENTS. 
. Introductory. 
. Means of distinguishing Intrusions and Extrusions. 
. General Features of Intrusive and Extrusive Sheets in Connecticut. 
. Special Accounts of the more important Localities. 
. Conclusions. 
oR oN Re 
1.— Introductory. 
THE outcrops of conglomerate, sandstone, and shale in the Triassic 
formation of the 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 deformations 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. 
2 Amer. Journ. Science, XXIV., 1882, p. 347. Bull. Museum Comp. Zodl., 
Geol. Ser., I. 1885, p. 249. 
3 Amer. Journ. Science, XXXII., 1886, p. 842. Amer. Assoc. Proc., XXXV., 
1886, pp. 224-227. Seventh Ann. Rep. U.S. G. S., 1888. Bull. Museum Comp. 
Zo6l., Geol. Ser., IL, 1889, pp. 61-87. Amer. Journ. Science, XXX VIL., 1889, pp. 
423-434. Meriden Scient. Assoc. Proc., 1889. 
VOL. XVI.— NO. 6. 
