PORPHYRY. 



85 



the rock by waves, and also its interposition in the slate, makes it very clearly a rock which 

 has been formed at a period subsequent to the consolidation of slate with which it is comiected, 

 and not by the successive depositions of particles from suspension in a watery medium. The 

 apparent stratification is interesting in another point of view : it proves the hability of mis- 

 taking this structure for a true stratification, in tliose cases where the means of detecting and 

 correcting the error does not exist ; as serpentine, basalt, greenstone, etc., all of which some- 

 times exhibit divisional planes analogous to those exhibited by the porphyry at Cannon's 

 Point. 



a. Slate ; i, Porphyry ; c. Disturbed portions. 



The wood cut, (fig. 20,) represents the position of the porphyry at Cannon's Point. The 

 disturbed portion is seen on the left, and the regular columnar mass in the middle and upon the 

 right. What is particularly worthy of attention, is the slight alteration and disturbance which 

 has followed from the injection of so large a mass of matter as appears in the slate at this 

 place. 



In addition to what is seen of this rock along the shore of the lake, it appears in conside- 

 rable force in the fields on the west side of the road, where it covers a hundred acres or more, 

 being apparently spread out over the slate and a portion of the Trenton limestone. It is quite 

 broken and fragmentary in many places, presenting low cliffs, in the face of which there are 

 a few rounded concretions. To the south about four miles, at a place known in the neighbor- 

 hood as Rattlesnake's den, it appears in a bluff of from one Imndred and fifty to two hundred 

 feet high. At this locality, it presents the same characters as at Cannon's Point ; and in 

 consequence of its structure and its exposure, it has been broken into an immense quantity of 

 pieces three or four inches in length and two in diameter, which have fallen down to the shore 

 and formed a steep talus, over which it is extremely dilEcult to pass, in consequence of the 

 loose state of the fragments which continually slide under the foot. 



Nature, Origin and Period of Formation of the Poi-phyry. 



If the porphyry appeared only at the last mentioned place, it would have been impossible 

 to have spoken very definitely of the period when it was formed, as the rocks in which it is 

 inclosed belong to the primary class ; but taken in connection with the slate of the Champlain 



