MUSEUM OF COMPARATIVE ZOOLOGY. 63 



out the structure of the anterior valley, the observer should return to 

 the anterior or amygdaloidal ridge and follow it northward about a third 

 of a mile from the ash cliff; here a trap-conglomerate (7) replaces the 

 ash and lava bed, and gives additional evidence of the contemporaneous 

 extrusive origin of the volcanic rock. Thin beds of sandstone occur in 

 the conglomerate, with strike and dip conforming to the general mono- 

 cline. Half a mile farther north, the lava again appears and, in its 

 northernmost outcrop (8), forms a bluff twenty to thirty feet high in a 

 spindle-shaped space between two roads. From this point, one may 

 renew the cross-section work by going eastward and climbing over the 

 thick bed of lava that forms Lamentation Mountain (9) ; but the result 

 is accomplished more easily by following the road, which lies chiefly on 

 a drift plain, and passing over the low north end of the mountain on a 

 level (10), until Spruce Brook is reached coming from the eastern slope 

 of Lamentation and running north. Crossing a field a hundred yards to 

 the south, the stream is found emerging from a little rocky channel 

 (11), cut in the uppermost part of the Lamentation trap sheet, and run- 

 ning down over the sloping beds of sandstone that rest upon it. The 

 strata dip and strike in accord with the general monocline. Close to 

 the trap they contain numerous vesicular fragments derived from it ; in 

 some of these, the vesicles may be seen to contain little deposits of 

 sandstone. It is an ideal " locality." A pool in the vesicular portion 

 of the trap, shaded by overhanging trees, tempts one to a bath after a 

 dusty walk. A little farther east, a ledge of brown shaly sandstone 

 (12) crosses the road, with normal strike and dip ; its outcrop persists 

 for some distance north and south at frequent intervals. Half a mile 

 yet more to the east, and at the further base of an immense drumlin, 

 known as Stow Hill, another small trap ridge (13) is discovered; this 

 may be called the posterior ridge, following Percival's nomenclature. 

 The points thus far described furnish about as much work as one may 

 care to do on foot in a summer day. 



Returning along the eastern side of Lamentation, the posterior ridge 

 may be followed southward with little interruption to its end, just be- 

 yond the Cromwell railroad. Where the railroad crosses it, a small but 

 valuable contact of the lava is exposed with the overlying sandstone, 

 which here contains an abundance of trap fragments for three or four 

 feet over the back of the trap sheet, clearly indicating the extrusive 

 origin of the sheet. The vesicles in some of these fragments are more 

 or less completely filled with sand, and under the glass the bedding of 

 these little deposits is seen to conform closely with the stratification of 



