TRIASSIC PERIOD. 489 



presence of few species that are properly marine. These facts 

 prove that the ocean had imperfect access, where any, to the re- 

 gions, — that the beds are not sea-shore formations like the Creta- 

 ceous and Tertiary of later times ; and thus they confirm the idea 

 that the beds are partly of estuary and partly of lacustrine origin. 

 The occurrence of vegetable remains and the coal beds sustain 

 this conclusion. 



III. The ripple-marks, raindrop-impressions, and footprints. 

 These show, wherever they occur, that the layer was for the time 

 a half-emerged mud or sand flat ; and, as they extend through 

 much of the rock, there is evidence that the layers in general 

 were not formed in deep water. They abound especially in the 

 upper half of the Connecticut-valley strata. 



IV. The thickness, — 3000 to 5000 feet or more. We learn from 

 this thickness, in connection with the preceding, that the areas 

 underwent a gradual subsidence of 3000 to 5000 feet or beyond ; 

 consequently, that these oblong depressions made at the time of 

 the foldings were slowly deepening, and continued to deepen until 

 the last layer was laid down. 



V. The tilted and displaced condition of the beds, without evi- 

 dence of folds. This inclination has been attributed to deposition 

 on a sloping surface. But such cases of oblique deposition are 

 exceptions, and not the general rule ; while in the case of the 

 sandstone, the layers are inclined 10 to 30 degrees or more, in each 

 of the great regions. The tilting must, therefore, be a result of 

 mechanical force ; and, as faults are not numerous, while joints are 

 common, it follows that the force was very gradual in its action. 



Under IV., a profound subsidence was shown to have been in 

 progress in the regions of depression occupied by the strata. Such 

 a subsidence would have brought a strain upon the overlying beds, 

 and sooner or later would have produced fractures and disturbance ; 

 and if one side or part of the depression were undergoing more 

 subsidence than the opposite, it would have caused that oblique 

 pushing of the beds that would have ended in faulting and tilting 

 them. The direction of the dip and strike in such a case would 

 depend on the relative positions, with reference to the whole basin, 

 of the parts undergoing greatest and least subsidence. 



VI. The sandstone strata intersected by dikes of trap. These 

 dikes are proofs of fracture of the earth's crust ; of more fractures 

 in the part of the crust directly beneath the formation than out- 

 side of the region ; therefore of fractures in the old synclinal 

 depression in progress of subsidence. The subsidence of such a 

 region would bring increasing tension or strain upon the rocks 



