104 BULLETIN OF THE 



hundred miles in length, the number of exposures upon them is very 

 small. Upper contacts are generally found in streams that descend the 

 back of the ridges, and these have therefore been examined most carefully. 

 The list below embraces all that we have yet discovered. The localities 

 are numbered to correspond with the figures on the map of Plate I., 

 and are arranged according to the sheet to which we suppose them to 

 belong, beginning with the trap range near the western border of the 

 formation, and proceeding with the anterior, main, and posterior sheets 

 farther east ; these being interpreted as has been explained in earlier 

 articles.^ Some specimens from the Palisade Range of New Jersey, col- 

 lected in 1883, are described with those from the western range of the 

 Connecticut Triassic. The several smaller ridges, not correlated with 

 any of the sheets above named, have not been closely examined, and are 

 not here referred to, except iu locality 26. The pages in Percival's 

 Report on the Geology of the State, where he describes the localities here 

 mentioned, are added to our list, for the sake of convenient reference. 

 Our descriptions are made as concise as possible, in order to shorten the 

 necessary repetitions ; several of the more interesting and instructive 

 localities are given more space in special accounts further on. Mention 

 is made in certain cases of peculiarities of structure that do not bear 

 directly on the question under investigation, partly in order that ob- 

 servations might not be lost, and also in the hope that the details thus 

 collected might iu time lead to new generalizations. Certain micro- 

 scopical variations in the trap naturally resulting from differences in the 

 conditions of solidification are added to those which have a direct bear- 

 ing on the question of origin ; not that they are criteria in themselves, 

 but that they have become recognized as commonly accompanying the 

 two kinds of eruption. For example, the occurrence of porphyritic 

 crystals in an eruptive rock does not establish its extrusive origin, but 

 extrusive sheets are notably more porphyritic than those solidifying 

 beneath the surface. So, too, a holocrystalline structure does not war- 

 rant us in saying that a rock is undoubtedly intrusive ; but intrusives 

 are more frequently holocrystalline and extrusives more frequently 

 glassy.2 But we have not attempted to give a complete petrographic 

 account of the specimens that have been examined. It seems advisable 

 to postpone this until samples from all the Triassic basins of the Atlan- 

 tic slope can be studied together. 



1 Seventh Ann. Report U. S. G. S., 1888. 



2 See, on the otlier hand, the account of recent lavas from Kilauea, in which glass 

 is rare or wholly absent. E. S. Dana, Amer. Journ. Science, XXXVII., 1889, p. 461. 



