432 MESOZOIC TIME — REPTILIAN AGE. 



sloping back. In many cases it occurs simply as a narrow dike. 

 It has come up through fissures in the sandstone, and, as it escaped, 

 it often thickened up into high elevations ; yet nowhere does it 

 seem to have flowed far over the surface. In many cases it has 

 made its way out by opening the layers of sandstone ; and, owing to 

 the direction of the dike or fissure, and that which this lateral 

 escape was calculated to produce, the ridge of trap has often as- 

 sumed a curved form, as apparent in the map. 



The proofs that the trap was actually melted are abundant. For 

 the sandstone rocks have in many places been baked to a hard grit 

 by the heat, and at times so blown up by steam as to look scoria- 

 ceous. In some places the uplift has opened spaces between the 

 layers, where steam has escaped and changed the clayey sandstone 

 into a very hard rock looking like the trap itself. Occasionally 

 crystalline minerals, as epidote and tourmaline, are among the 

 results of the baking. The evidences of heat, moreover, diminish 

 as we recede from the ridges ; and there is no doubt that the sand- 

 stone has been extensively worn away by waters where it had not 

 been rendered durable by the heat. 



In all the several regions along the Atlantic border the strata are in most 

 parts much tilted. In North Carolina there is in general a dip of 10° to 22° 

 to the southwest (Emmons) ; in Virginia, Maryland, Pennsylvania, and New 

 Jersey, the dip is to the northwest or north-northwest (Rogers) ; in Connecti- 

 cut and Massachusetts, to the east or southeast (Hitchcock). But there are 

 many variations at short intervals. In the Portland quarries there are joints 

 on a grand scale, having two transverse courses, nearly north-and-south and 

 east-and-west. 



Some of the dikes of trap and fissures in the sandstone in Con- 

 necticut and New Jersey contain copper-ore (copper-glance, eru- 

 bescite and malachite), and there is little doubt that the copper 

 veins, and the barytes which is often the gangue of the vein, ori- 

 ginated in the same period of eruption. The red color of the 

 sandstone — a consequence of oxydation of magnetic-iron grains 

 present in it — appears to have had its origin in the same cause. 



This history of the Triassic of the Atlantic border and its trap 

 dikes appears to be a repetition of what took place long before, 

 during both the Huron ian and Potsdam periods, in the Lake 

 Superior region, where a similar subsidence (10,000 feet in the 

 former, and 3000 or 4000 in the latter), and similar igneous erup- 

 tions, accompanied the formation of the beds. 



