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 L., 
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 
anv of the sheets above named, have not been closely examined, and are 
not here referred to, except in locality 26. The pages in Percival’s 
teport 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 in 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. 8S. G. S., 1888. 
2 See, on the other 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. 
