808 HISTORICAL GEOLOGY. 



It is a fact deserving especial note that although the subterranean fusion 

 occurred at intervals for 1000 miles, the fissures by which ejections took 

 place were almost wholly confined to the narrow areas of the Triassic 

 geosynclines. The isolated Southbury area of Connecticut, a dozen miles 

 west of that of the Connecticut valley, and only seven by two and one half 

 miles in area, has its many trap dikes ; and none exist over the intervening 

 region or to the north, west, or south of it. The isolation of the eruptions 

 corresponds with that of the upturning. The areas of the geosynclines — that 

 is, of subsidence and deposition — in some way localized the areas of fractures 

 and fusion. There seems to be good reason, in the facts, for locating the 

 chief of the fractures underneath the center or central line of each area 

 and under that half of it which is nearest to the general axial line of 

 the chain of areas, rather than underneath the outer margin of the other 

 half, or in any part of this half ; that is, in the Connecticut valley area, as 

 Percival's map illustrates, for locating it underneath its central line and the 

 half to the westward, rather than underneath the eastern part. 



There are, however, two long dikes just east of this Triassic area, 

 besides two others to the west of it. One of the southwestern of these out- 

 side dikes, bb on the map, is proved, by its cutting through the West Rock 

 trap, to have been of subsequent origin ; and this is probably true of all four. 

 The four are alike, moreover, in having a mean course of N. 25°-30° E., thus 

 differing about 15° in easting from the average course of the trap belts in 

 the Connecticut valley. Similar facts are afforded by the region of the Pali- 

 sade Range. They accord with the idea that these outside dikes were 

 erupted when the orogenic catastrophe was near its close, and the localizing 

 geosynclinal conditions had lost part of their influence. Perhaps tension 

 from a decline in the lateral thrust, or from a dissipation of the subterranean 

 heat generated by the movement, led to these divergent lines of fracture 

 and eruption. 



According to J. J. Stevenson, great displacements have been produced 

 in the faulted Appalachian region of northwest Virginia at some time 

 subsequent to the origin of the range; and it is probable that the epoch 

 was coincident with that of the Triassic upturning. (Am. Jour. Sc, xxxiii., 

 262, 1887.) 



Movements over the Rocky Mountain Region and the Pacific Border. 



Making of the Sierra Nevada. 



Along from Mexico northward, in the Rocky Mountain region, thick 

 Triassic and Jurassic deposits were in progress, preparatory for future 

 mountain-making; but, in general, only oscillations, and some emergences in 

 the general course of geosynclinal subsidence, have been reported. Over 

 the summit region of the Rocky Mountains deposition was continued quietly, 

 as a general thing, through another period, the Cretaceous, before any great 

 disturbance took place. On the ground of the absence of Liassic beds over 

 the region south of Wyoming, R. C. Hills has inferred that an emergence 



