278 Prof. J. D. Dana on some Results of 



movements and flexures of the crust, and the opening of fissures 

 for the escape of the mobile rock, great regions of it may have 

 been pressed up and become isolated fire-lakes at a higher level. 

 Or, as Professor Hopkins suggests, the subterranean sources of 

 volcanic action may have been in shape like inverted cones ex- 

 tending far up into the crust, and may have finally been divided 

 off into " smaller ones by a process similar to that by which the 

 larger cavities were themselves formed from the general fluid 

 mass, the position of each minor cavity being determined by the 

 points in the roof of the larger one, where previous fractures had 

 left the most perfect volcanic vents." This last explanation 

 holds if regions of volcanic vents date back to early geological 

 time, so that modern volcanoes in an area are successors to in- 

 definitely older ones, but is less applicable where the vents of 

 a region are of Camozoic origin (as appears to be true of most of 

 them) and a consequence of the later movements of lateral 

 pressure. 



The inference that igneous eruptions have generally been 

 derived from a deep-seated source is sustained by the great 

 lithological uniformity of ejections over widely distant ranges of 

 surface. The eruptive rocks (or trap) of the Triassico-Jurassic 

 areas of the Atlantic border from Nova Scotia to the Carolinas, 

 already many times referred to*, all which belong to one epoch, 

 are solely varieties of dolerite — rocks made up essentially of 

 Labradorite and pyroxene, with more or less of magnetic iron- 



* These hills and dikes of " trap " (as they are ordinarily called) have 

 great length and breadth in Nova Scotia. In the Connecticut valley they 

 are very numerous, and in many lines — as laid down, especially for Con- 

 necticut, by Percival in his ' Geological Report.' A copy of a part of his 

 very accurate detailed map of the trap-region is reproduced in the writer's 

 ' Manual of Geology,' page 20. At the east and west bend in the dikes, 

 south of the middle of the map, are the Hanging Hills of Meriden, 900 to 

 1000 feet high, situated about 20 miles north of New Haven (and 12 miles 

 from the southern margin of the map) ; and the range which extends from 

 these hills north continues to Mount Tom, near Northampton, in Massa- 

 chusetts. The range to the east, south of this bend, continues southward 

 to Lake Saltonstall, east of New Haven. Other lines cut through the me- 

 tamorphic rocks : one of these lines of trap-ejections in the eastern meia- 

 morphic region of Connecticut, as mapped by Percival, extends from Long- 

 Island Sound, 6 miles east of New Haven, nearly to the southern boundary 

 of Massachusetts, over 70 miles, with a nearly east-north-east trend, 

 consisting of an interrupted series of dikes, intersecting metamorphic 

 rocks of various kiuds. A second independent Triassico-Jurassie trough 

 exists in Connecticut, 15 to 20 miles west of the main one, over part of 

 the towns of Southbury and Woodbury ; and this also has its numerous 

 trap dikes. The Palisades of New Jersey, bordering the Hudson River, 

 are the northern part of a complex series that continues southward and 

 westward through New Jersey and Pennsylvania into Virginia, following 

 the course of the Triassico-Jurassic sandstone areas. There is also a series 

 in the North-Carolina area. 



