MUSEUM OF COMPARATIVE ZOOLOGY. 295 



The trap sheets which are definitely determined to be intrusive show 

 no vesicular structure at any point so far as I liave seen them ; they are 

 dense throughout, fine at the margins and very coarse in the centre of 

 the larger sheets. Their metamorphic effect on the adjoining sandstones 

 is very marked, as will be described below. The even intrusion of these 

 sheets between shaly or sandy strata is very remarkable. At Martin's 

 Dock and Point Pleasant Station, N. J. (N, 0, figs. 51, 52), where this 

 is best shown, trap sheets of vai'ious thicknesses, from two to twenty 

 feet, are seen evenly interposed between the strata above and below 

 them for fifty or more feet, without breaking across the layers at any 

 point : in two cases the slaty partings between adjoining trap sheets 

 are less than one foot thick, and yet are continuous for over fifty feet. 

 From this it must be supposed that the molten trap was injected slowly, 

 and that it acted as a liquid wedge, prying open a passage for itself along 

 the planes of easiest breakage that could be found : where two sheets 

 are close together, one was probably intruded after the other. Still it 

 is surprising that any rock could break so evenly as the Triassic strata 

 are thus proved to have broken. 



It is not a little interesting to discover that the largest clearly intru- 

 sive sheets, the AVest Piock range and the Palisades, are found on the 

 outcrop side of their respective sandstone belts; they are near what 

 would be the bottom of the sandstone series if the entire formation had 

 received a single monoclinal tilting. The intrusions on the Delaware are 

 also near the base of the formation, as is shown by the appearance of 

 the Matinal limestone not far from them. The probable cause of this is 

 suggested below. 



The date of these intrusions is indeterminate. It cannot be fully 

 shown when they took place ; whether at about the time of the over- 

 flows, that> is, the latter half of the Triassic time, or whether their 

 intrusion came later, when deposition was stopped by upheaval and dis- 

 location ; but the latter is the more common view. 



Professor Dana has already been quoted as taking the position of the 

 trap sheets as evidence that their intrusion came after the sandstones 

 had been tilted extensively (c, 421). But the laccolites of the Henry 

 Mountains, Utah, as described by Gilbert, and similar intrusive rocks in 

 Colorado described by Peale (referred to above) lie between horizontal 

 strata ; it is therefere not necessary that the sandstones should have 

 been tilted in order that the trap might be forced in between its layers. 

 Professor Dana further argues, from the fact that the trap columns on 

 the f:\ce of the ridges are at right angles to the sandstone layers, that 



