158 STRATIGRAPIIICAL GEOLOGY. 



to thirty inclics wide. The occurrence of this dyke at once explains the 

 origin of the chasm. The running water wears away the hght-colored 

 trap. Then water, percolating the vertical joints back of the eroded 

 space, freezes in the cold weather, and, by the consequent expansion, 

 pushes a mass of rock inwardly till it falls into the stream. During the 

 warmer months the water is busily engaged in pulverizing the boulders, 

 assisted by chemical reagents, or the removal of the potash of the feld- 

 spar by solution. As soon as there is space for the further action of 

 freezing in the joints, other masses of granite are overthrown and worn 

 away, till Nature has succeeded in manufacturing the completed flume. 

 In the heliotype one sees on the right the remnants of some of the ver- 

 tical sheets not entirely broken down. It is likely that the regularity or 

 parallelism of the walls has been perfected by the action of the dyke 

 upon the rock when in the formative condition. 



The Pool is a circular excavation in the same granite, about one hun- 

 dred and fifty feet in diameter, and forty deep. It is supposed that it has 

 been excavated by the action of the Pemigewasset river, just as is the 

 case with the Basin, — a pot-hole about twenty-five feet in diameter, situ- 

 ated midway between the Flume and Profile houses, by the side of the 

 carriage-road. It is figured on the same plate with the Flume boulders. 

 The rock is the same as in the Plume ; but the Franconia breccia under- 

 lies it just above the junction appearing in the view. 



Very large exposures of the Conway granite show themselves in con- 

 nection with cascades on the stream from the west just below the liasin. 

 This granite shows orthoclasc and oligoclase. It extends southerly, cap- 

 ping Mt. Pemigewasset, this being its farthest development in that direc- 

 tion. At Tamarack pond and over Profile mountain the same feldspar, 

 with quartz and mica, are present ; but the materials will not average 

 more than the tenth part of their lengths in the coarser rock. The 

 north-west spur of Profile mountain, towards Franconia, is clearly a gran- 

 itic cone, probably like that of the principal part of the mass. The Pro- 

 file is made of similar materials, in a decomposed state. These ledges 

 often approximate to the porphyritic structure. The planes dip west of 

 north near the Profile house. Trap and porphyry dykes occur on the 

 same side. On the eastern summit of Profile the planes dip S. 35° E. 

 The highest part of the mountain, to the west of the location usually 



