802 HISTORICAL GEOLOGY. 



ee, another on the east ; S, Saltonstall Ridge, called Pond Ridge by Percival ; T T, 

 Totoket Ridge ; C, Mount Carmel ; M, Meriden ; ]\It, Middletown ; Pd, Portland and 

 Portland sandstone quarries ; H, Hartford. The scale of the map is j\ of an inch to 5 

 miles. The many interruptions in the lines of trap on Percival' s map are generally due 

 to intervals of sandstone, and the smaller of them may often have resulted from falls of 

 the sandstone walls of oblique fissures, as explained on page 298. But in some cases they 

 are breaks in the outcrop of trap in which no sandstone was in view, and where further in- 

 vestigation may prove the line to be continuous. One such case exists in the termination 

 of West Rock, and another in the south side of the summit of Mount Carmel ; and changes 

 have been made correspondingly in Percival's map. One other change made, in order to 

 represent the results of later observation, is the continuation of the dike bb to and across 

 West Rock. 



Some of the general facts of importanoe illustrated on the map are the 

 following : — 



1. The outcrops are most numerous in this southern portion of the area. 

 To the north of the region here mapped, there are only continuations of 

 the three western lines to Mount Tom and Mount Holyoke in Massachusetts, 

 and an isolated line farther north which passes near Greenfield. 



2. The outcrops of trap are not wholly confined to the Triassic area. 

 Two lines of dikes exist on the west side (jW) ^^> on the map) ; they con- 

 tinue southwestward to the Sound, In one of them, the trap is sparsely 

 porphyritic with crystals of anorthite. There are also two long dikes on 

 the east : one, commencing in ee, to the eastward of New Haven, not a mile 

 distant from the area, has a course nearly parallel to its eastern outline 

 for 10 miles, but afterward diverges from it ; the other commencing nearly 

 east of Hartford, just outside of the area, is parallel to the area for the 

 same distance. Both were traced by Percival to the Massachusetts line. 



The convergence of the dike ee, southwestward toward New Haven Bay, 

 and that of the other lines of trap in the Triassic area, are part of the 

 evidence that the estuary or trough terminated at this place. 



3. The trap (doleryte or diabase) is essentially the same rock in all the 

 belts, and through all the Triassic areas. It is sparingly chrysolitic, 

 according to Iddings, in Orange, N. J., and rarely so in other places. 

 The chief variation is a result of alteration by means of water imbibed as 

 vapor, when, it is believed, the rock was on its way through the sandstone to 

 the surface. The rocks are sometimes unaltered on one side of a belt, and 

 much altered along its middle or on the other side. Dikes intersecting the 

 outside crystalline rocks are wholly free from the alteration, showing that the 

 moisture was not from the same source as the trap, but more superficial. The 

 altered hydrated trap has little luster ; is often amygdaloidal within 50 feet 

 or so of the surface ; and decomposes rapidly, and often to a depth of several 

 yards, so that a small dike between layers of sandstone is sometimes found 

 wholly changed to a brownish yellow earth, and looks like a bed of tufa. 

 For remarks on amygdaloids, see pages 78, 336. Along some of the fissures 

 there were carried up with the trap ores of copper, and thus copper veins 

 were made in the trap and in the sandstone of the vicinity (page 338). 



