TRIASSIC PERIOD. 421 



long era, and finally, in the Triassic era, had been swept off from the 

 land into the valley, by the flood referred to. 



V. The thickness, — 3,000 to 5,000 feet or more. — We learn from 

 this thickness, in connection with the fact just stated, that the areas 

 underwent a gradual subsidence of 3,000 to 5,000 feet or more ; con- 

 sequently, that these oblong depressions made at the time of the fold- 

 ings were slowly deepening, and continued to deepen until the last 

 layer was laid down. 



VI. The tilted condition of the beds, without evidence of folds. — The 

 tilting must be a result of mechanical force ; and, as the bedding is 

 well preserved, while joints are common, it follows that the force was 

 very gradual in its action. Under V., a profound subsidence is stated 

 to have been in progress, in the regions of depression occupied by the 

 strata. Such a subsidence would have brought a strain upon the 

 rocks of the trough below, 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. 



VII. The sandstone strata intersected by dikes of trap. — These 

 dikes are proofs that fractures took place. The subsidence of such a 

 region would have brought increasing tension or strain upon the rocks 

 below, tending to produce fractures, especially about the axial region 

 of the depression. Either thus, or as a direct result of the lateral 

 pressure, openings were made for the escape of the melted rocks. 

 See, further, pp. 801, 803. 



The manner in which the trap at its eruption has sometimes sep- 

 arated the layers of sandstone, and in this way escaped to the surface, 

 instead of coming up through the fissures simply, shows that the rock 

 had been tilted extensively before the ejection. 



In the north-and-south ridges of the Connecticut valley, the trap which thus escaped 

 now shows, as already observed, a bold front to the westward, the dip of the sandstone 

 being to the eastward. Now, in this bold columnar front, the angle of inclination in 

 the columns is just the angle of dip in the sandstone, the columns being at right angles 

 to the layers of sandstone. Hence, the inclination in the sandstone layers existed before 

 the time of ejection, and determined the position of the columns; for the columnar 

 structure of trap io always at right angles to the cooling surfaces; and these surfaces 

 were those of the opened layers of sandstone. We have proof therefore that there was 

 a tilting of the strata in progress, before the final breaking and ejections. 



Era of the Eruptions of trap. — As the trap dikes intersect the later beds of the for- 

 mation, the igneous ejections must either have been among the closing events of the 

 sandstone period, or have occurred in a succeeding epoch. 



Thus the period of these rocks came to a close somewhat similar to 



