MUSEUM OF COMPARATIVE ZOOLOGY. 2G1 



area, as above suggested ; although, so far as the origin of the trap is 

 concerned, it is immaterial whether the ridges are regarded as a single 

 sheet fixulted or as separate sheets. 



B. Mount Tom, Mass. (figs. 35, 36, 37). — This section is one of the 

 best I have seen. The following description is reversed from the de- 

 scending order in which the observations were made. The bench on 

 the western face of the mountain below the talus of trap fragments is 

 dependent on a hard sandstone conglomerate ; it forms a ridge by itself 

 from the southern end of Tom over two miles to Rock Valley, where it 

 dies away. The rock quarried at Mount Tom Station, Connecticut River 

 Railroad, seems to be of the same horizon. From this bench the sand- 

 stone is generally hidden by trap blocks ; but about three quarters of a 

 mile north of the end of Mount Tom, there is a small exposure of sand- 

 stone in contact with the great trap cover; others doubtless occur, but 

 on finding this one I searched no farther. The sandstone here, a few 

 feet from the trap, is gray, micaceous, and clearly bedded ; but near and 

 at the contact it is dense, as hard as quartzite, bluish, and somewhat 

 like the ti'ap itself in appearance, with only the larger bedding-joints 

 remaining. The two rocks are not welded together here : an open seam 

 separates them, so that no single specimen shows both. The trap is 

 dark, dense, fine-grained, and excessively hard ; it lies evenly on the 

 layers below without cutting them ; the columnar structure does not 

 show for some ten feet higher. Along the mountain summit the texture 

 of the rock is much coarser, the crj^stals are occasionally a tenth of an 

 inch in length. Descending the long eastern slope by a wood road, there 

 was no clear evidence of any persistent variation in form, such as occurs 

 by the Poet's Seat in Greenfield, or on the back of the First Mountain, 

 south of Paterson, N. J. The entire mountain seems to be a single 

 heavy trap sheet, several hundred feet thick. When nearly at the 

 bottom of the valley, between the main and the posterior I'ange, an 

 excellent series of exposures was found in a little gully leading down 

 the slope. First there was a glaciated surface of firm trap. Descend- 

 ing a little farther (ascending in the geological series), there was found 

 some forty feet of ragged, rough surface, in part trap, and in part clearly 

 a mixture of angular trap fragments with sandstone. The surface of 

 the trap is often vesicular, and in places shows included fragments of a 

 denser kind. The sandstone is reddish, not nearly so hard as that found 

 on the western face of Mount Tom, though harder than some of the soft 

 fragments of trap, which weather out leaving a rough framework of 

 sandstone; at some points the bedding can be seen, but exposures are 



