366 E. O. Ilovey — Trap Ridges of the 



The western subordinate ranges. — Of the three ranges of 

 trap ridges mentioned as lying west of Pond Rock, the one 

 nearest to the main ridge is the most continuous. Beginning 

 at the railroad, a third of a mile west of the Pond ridge, this 

 range (W 1, on the map), extends to the north and northeast 

 for rather more than four miles. The convexity toward the 

 west is marked, though not as noticeable as that of the main 

 ridge ; like that of the Pond ridge at the southern end, it is 

 caused by the exposures of trap being arranged in what Perci- 

 val calls the advancing order ; i. e., the southern end. of the 

 northern member overlaps on the west the northern end of the 

 southern member. The northern end of the range consists of 

 a very complex set of short ridges which may, perhaps, be joined 

 into two or three strongly curved ranges. Each of the other 

 two ranges (W 2, and W3), is composed of many small exposures, 

 but the most western has several large ridges, the largest of 

 which are Hemingway mountain, 260 feet high, and Eaton 

 Hill, 200 feet high, north of the borough of Fair Haven East, 

 and two nameless hills, each 240 feet in height, north of and 

 in line with Hemingway mountain. Besides these ranges of 

 ridges there are many small exposures of trap in this part of 

 the district which do not seem to belong to any range. 



Kinds of trap rock in different parts of the region. — Dr. G. 

 W. Hawes* and Prof. E. S. Danaf made a somewhat extended 

 chemical and microscopic study of the trap rocks of the Con- 

 necticut valley some years ago. The results, though incom- 

 plete, show that the trap is of very uniform mineral composi- 

 tion, and that the main differences are due to the alteration 

 consequent upon hydration. These observers concluded that 

 the hydration increased from west to east, the trap of the 

 western border of the Triassic being almost non-hydrous. 

 While the increase is not a regular one, it is true that the 

 ridges of the eastern part of the valley generally show more 

 alteration than those of the western. The trap is a true dia- 

 base, the augite of which has suffered the most from alteration. 

 The alteration products, according to my study of slices, are 

 usually chlorite and magnetite, the latter sometimes arranging 

 itself in beautiful arborescent groups of crystals, while the 

 former frequently exhibits a fibrous structure. The ridge 

 which shows the most complete alteration of any in the region 

 is a part of the most western of the six ranges (W3). Where 

 exposed by Farren avenue and Center street the rock has been 

 deeply decomposed, while other ridges in its vicinity and 

 Hemingway mountain, a mile and a half north in the same 

 range, are composed of hard dense trap, not very lustrous but 

 still showing some of the augite unaltered. The trap of the 



*This Journal, III, ix, 192, 1875. f This Journal, III, viii, 390, 1874. 



