1883.] 435 [Crosby. 



of the town, and is cut out of the southwestern flank of a broad, 

 rocky, and wooded hill. This chasm is quite straight, about 

 one-fourth of a mile long, fifty feet wide, and has a northeast 

 and southwest trend. The walls are vertical, vary in height 

 from ten to seventy feet, and are composed of a massive, mica- 

 ceous gneiss, which is the prevailing rock of that region. The 

 bottom or floor of the chasm is encumbered to an unknown depth 

 with large, angular masses of the gneiss. The gneiss dips to the 

 northeast about 25°, and is traversed by two well marked systems 

 of joints ; one system running northeast and southwest, or paral- 

 lel with the chasm ; and the other in a direction at right angles 

 to this. The gneiss is also intersected by numerous veins of en- 

 dogenous granite, holding beryl, garnet, and other minerals, 

 which coincide in trend with the joint planes. 



The only explanations of the origin of the Sutton purgatory 

 which I have seen are the two suggested by Dr. Edward Hitch- 

 cock in his Report on the Geology of Massachusetts. The first, 

 which he rejects as being too violent and as unsupported by the 

 disposition of the gneiss, which does not dip away from the 

 chasm, supposes the chasm to have been produced during a local 

 uplift of the strata. 



According to the second hypothesis, which Dr. Hitchcock was 

 inclined to accept, this chasm is similar in its origin to the Pur- 

 gatory at Newport, R. I. ; i. e., it is the product of marine erosion 

 at a time when the sea occupied the surrounding valleys. 



It appears to me, however, after an examination of the ground, 

 that the second supposition is even more untenable than the first. 

 At no point do the walls of the chasm show the slightest trace of 

 water action ; and the floor, instead of being bare and smooth, or 

 covered only with rounded and water-worn boulders and pebbles, 

 is piled with huge, angular blocks of gneiss. The same facts, as 

 well as the situation of the chasm on a hill and not in a valley, 

 are fatal to the notion that it is the channel of some ancient 

 river. Glaciation, also, is entirely out of the question, for the 

 chasm is transverse to the direction of glacial movement, and its 

 walls are entirely unglaciated. 



The only explanation commending itself to my mind is that 

 referring the chasm to a local subsidence. In short, I conceive 

 that during some disturbance of this portion of the earth's crust, 



