THE POSTEEIOE SHEET. 467 



Just northwest of the Snuths Ferry raih-oad station the trap is fauhed 

 shghtly, the south side being moved a few rods westward, and in the low 

 pUice in the ridge thus formed the sandstone approaches within 80 rods of 

 the raih'oad. 



THE GREAT WIDENING OF THE TRAP AREA AND THE FEEDIVG THROAT 



BENEATH. 



A mile south of Smiths Ferry the trap widens to a triangular surface, 

 a half mile on a side, and the ridge reaches its greatest height, rising 

 westerly from the river to its crest in Little Mountain (now marked ])y 

 the highest lookout tower in Forest Park) and sinking by a vertical wall to 

 the valley which separates it from the Mount Tom trap ridge. (See PI. 

 IX, p. 446, northeast of Mount Tom.) Along the western edge of the 

 expanded area the thin trap sheet still rests normally on the sandstone, 

 and on the eastei-n edge is covered by the tuff, and its great width is due 

 to the fact that it dips with the slope of the hill east from its crest. In the 

 deep inlets of sandstone running down into the trap from the north the 

 latter can everywhere be seen to lie normally on the sandstone, with little 

 baking, and along the border from this point around to the west the same 

 conditions hold for a long way south, until one comes to the point where 

 the wood road coming up from the reservou- crosses the bi'ook and goes up 

 onto Mount Tom, and where the posterior ridge itself rises to its greatest 

 height in Little Mountain. At this point the face of the core is finely 

 exposed for study, as indicated in fig. 25. The trap comes up from the 

 depths with but a small portion (30 feet) of its width exposed, sending 

 out great dikes into the sandstone north and south. The southern dike, 

 starting with a width of 8 feet, was followed 50 feet. 



North of the core a small dike is seen inclosed wholly in the sandstone, 

 and a wide dike branches from the main mass and can be followed a long 

 way north before it is concealed by the talus. At the surface the trap flows 

 out over the sandstone, greatly indurating it, and becomes the sheet which 

 we have followed from the north to this point. The whole is like a great 

 toadstool; the stem is the core which forms Little Mountain. The west- 

 em and most of the southern part of the "umbrella" is broken off by 

 erosion ; the eastern part is the sheet dipping east beneath the tuff. 



The exposed wall of trap sho^'^■n on the left in the figure seems to be a 



