JOINTED STRrCTTRE. 



31 



The separatiim of tlic rock by ihe na- 

 tural joints into blocks, often gives the 

 rock a peculiar aspect. The annexed 

 figure will serve to give an idea, both of 

 those joints, and the elTccts of weathering. 

 The appearance produced is not uniform ; 

 the surface of the rock is frequently 

 checked without nuicli regularity. It is . 

 also veined, or traversed by veins of se- 

 gregation, which are merely harder por- 

 tions of the same materials which com- 

 pose the rock ; and for this reason they 

 resist the action of the weather, and hence appear in relief upon the face of the rock. They 

 thus appear like true veins, and give the face of the rock a chequered appearance. Besides 

 the veins of segregation, and the seams marking the course of the natural joints, cracks ap- 

 pear without regularity, which serve to divide the rock into wedge-form pieces, and which in 

 fact are finall}"- broken out, and are strewed around the base of the cliff. Tn this respect, the 

 rock has a strong resemblance to greenstone trap, or rather sienite, excepting that the latter 

 is a stratified rock. 



The rocks which are associated with the one under consideration, arc those allied to trap, 

 of which the most constant in its occurrence is a compound of hornblende and feldspar, form- 

 ing a granular mass quite friable in its texture, and of a dirty grey color. It is generally 

 found in layers in the veins of iron ore, or in the vicinity of ore beds ; it often forms, also, 

 dykes traversing the rock in the manner of the common greenstone trap. An instance of this 

 may be seen at Avalanche lake, where an enormous dyke cuts through Mount McMartin. A 

 deep and almost impassable gorge, formed by the breaking up of this mass, extends to the 

 summit of the mountain. 



Two or three varieties of porphyry have been noticed also in this rock. In one of these 

 varieties, the base or ground is a light green compact feldspar, spotted with small masses of a 

 deep bluish green feldspar. It forms quite a handsome rock, and would polish extremely 

 well. It has, however, too much of a mottled appearance ; the small masses being rather too 

 indeterminate, or not in sufficiently distinct crystals to constitute a true porphyry. Another 

 kind is white and reddish white ; it appears like a mass of breccia. The gi-ound is a compact 

 feldspar, either white or flesh-red, in which there are angular masses of gi-cenish compact feld- 

 spar. Connected with the two last rocks is prehnite, in mammillary masses of a pale green 

 color, with a few indistinct crystals. It is one of the rare instances of the occurrence of an 

 earthy mineral in the primary rocks, belonging to the family of the zeolites, or kouphone sjjars. 

 It is found on the East river, about three miles from the settlement at Adirondack. Indistinct 

 seams of prehnite are, however, not uncommon ; but they are so imperfect that they will 

 generally escape attention. A locality in the town of Keene furnished a few fine specimens ; 



