MUSEUM OF COMPARATIVE ZOOLOGY. 81 



West Peak sheet. The trend of the western face of Notch Mountain is 

 N. 50° E., and it may be traced in a more or less distinct bluflF or ledge 

 (12) for a mile in this direction. On following up the intercepting 

 canal cut in the trap to bring the streams from the back of West Peak 

 into the north end of the reservoir, a band of breccia about a foot wide 

 may be found traversing it, trending N. 60° E. ; and the back of West 

 Peak is furrowed with ravines (13) trending N. 55° or 60° E., showing 

 that faults of the normal direction occur here as well as to the east of 

 the reservoir. The general topography leading to the above conclusion 

 may be perceived from a knoll (15) a good half-mile northward. The 

 long northwestern face of Notch Mountain is from here clearly seen to 

 be independent of the detached portion (14) of the West Peak block. 

 A low trap ridge (16), a little to the west of the knoll, is probably to 

 be identified as the posterior sheet of the same block. 



The abnormal position of the reservoir valley finds no sufficient ex- 

 planation. It may be located on a branch of the chief fault ; but in 

 such case the chief fault ought to be the site of the chief valley, and 

 not merely of a little ravine. Perhaps a more likely explanation will 

 some day be found' by regarding the reservoir valley as the abandoned 

 course of an old river, whose direction was taken during the pre- 

 cretaceous base-levelling of the region, and maintained for a time after 

 the post-cretaceous elevation, until some other stream, which en- 

 countered no heavy trap sheet and therefore deepened its channel 

 quickly, captured and led away the head waters of the reservoir river ; 

 the reservoir notch would thus fall into the class of wind gaps derived 

 from water gaps, not uncommon in the Appalachians. But diflFerent ob- 

 servers may well have different opinions here. 



While on the knoll (15), the double form of Notch Mountain will be 

 observed. A second trap sheet (17) seems to lie on the back of the 

 first. (It is rather too distinctly drawn in Fig. 11.) The same thing 

 might have been noticed from the back of the anterior sheet of Cat Hole 

 block, where the roads form a little triangle. A rough walk through the 

 woods around the base of the upper sheet (17) to Cat Hole shows the 

 back of the lower sheet to be highly vesicular ; a rocky talus hides 

 the contact between the two. I have interpreted this as the topo- 

 graphic expression of the two lava sheets disclosed in the quarry blufi" : 

 the vesicular upper part of the lower sheet acts as a soft bed between 

 the denser parts of the two flows, and the mountain crest is therefore 

 doubled. The same double form may be seen in the mountains of 

 Medina sandstone in central Pennsylvania, and for a similar reason. 



VOL. XVI.— NO. 4. 6 



