178 UMRAIBIL, (Cp HOS SIBILIL 
the High Torne, in Rockland county, New York. The breadth 
of the surface now exposed by erosion, is from one to two miles. 
The central part of the sheet dips westward, in conformity with 
the enclosing strata, at an average angle of about 15°, but the 
dip increases toward the northern extension of the sheet, where 
irregularities occur. As shown by Darton, the dike that supplied 
the sheet, is exposed at a few localities on the western margin 
of the part now uncovered. The sheet does not, therefore, extend 
indefinitely beneath the sedimentary beds to the westward, as 
was at one time supposed. The Palisade sheet is remarkable for 
its extent and thickness, but except in these features, does not 
illustrate the facts to which I wish to direct attention, so well as 
many smaller examples of the same nature. In other portions 
-of the Newark system, especially in the Connecticut valley, 
much thinner intrusive sheets occur, and many similar examples 
in other regions are familiar to most geologists. One of the 
best illustrations of the manner in which molten rock has been 
forced in. between the strata of horizontally-bedded sedimentary 
rocks, which I have observed, may be seen in the precipitous 
walls of the canyon excavated by Purgatory River in Cretaceous 
terranes, in southeastern Colorado. The beds cut through in 
forming this canyon, include a sheet of basaltic rock four or five 
feet thick, which is exposed for three or four miles on each side 
of the gorge; so perfectly does it conform with the strata above 
and below, that even a careful observer seeing its outcropping 
edge for the first time from the bottom of the canyon, would not 
mistrust its intrusive origin. 
In the case of the intrusive sheet exposed in Purgatory canyon, 
as already stated, the enclosing strata are horizontal. The same 
is true of similar sheets in many other regions. It is supposed 
with good reason that the intrusive sheets of the Newark system 
were forced in before the associated beds were tilted and faulted. 
The fact that intruded sheets are of frequent occurrence in 
regions where stratified rocks are yet horizontal is suggestive. 
™N. H. Darton, “The relation of the traps of the Newark system in the New 
Jersey region,” U.S. Geological Survey. Bulletin No. 67, p. 44. 
