yO STRATIGRAPHICAL GEOLOGY. 



that of Percy peaks or Sugar Loaf, but it resembles somewhat that at 

 Colebrook. The difference is chiefly in the color of the feldspar, that of 

 the Slide being light gray. The face of the Slide is a precipitous wall of 

 rock, from one to two hundred feet in height, above the debris that has 

 fallen down at the base of the cliff. The wall itself forms an immense 

 amphitheatre, — hence there is a fine opportunity for studying the rocks. 

 On the west, near the railway at the base of the cliff, the whole mass of 

 the rock is sienite; in places it is discolored, but generally it is composed 

 of a light-colored feldspar, of quartz that is light gray, and hornblende. 

 In places the hornblende is wanting, and elsewhere the rock seems to be 

 composed almost altogether of feldspar. This is particularly the case 

 towards the summit of the cliff. When we have passed two thirds of 

 the way around the amphitheatre, we find a schist. The strata are gene- 

 rally vertical, or nearly so, and they are in contact with the sienite ; in 

 places are penetrated by it. There is a similar arrangement of rocks on 

 the north-west side of Mill mountain. Towards the east side of the am- 

 phitheatre there are several dykes of diorite of considerable interest. It is 

 often amygdaloidal, and the cavities contain calcite. The heliotype shows 

 most prominently the west end of the Slide; and, although the whole 

 bluff is precipitous, near the centre of the picture can be seen the per- 

 pendicular wall of rock. On the extreme right is the point where the 

 sienite is succeeded by the schist. A close inspection will show the 

 Grand Trunk Railway, where it passes along at the base of the cliff. 



PORPIIYRITE.* 



Porphyrite is quite a common rock in northern New Hampshire. The 

 most northern locality where it is developed is in Stratford. East from 

 Connecticut river and south of Lyman brook, there is a range of moun- 

 tains, one of the highest points of which is known as Lightning mountain. 

 It can be seen from the vicinity of North Stratford, and it is the only 

 peak in the range that is without trees. On the south side of this moun- 

 tain there is quite an area where there is an exposure of a dark granitic 

 rock, composed chiefly of feldspar, which gives to it a greenish color; 

 there is also in the rock a small proportion of a dark vitreous quartz. 



* Rocks classified and described by 13. Von Cotta. English edition, by P. H. Lawrence, p. i68. 



