86 GEOLOGY OF THE SECOND DISTRICT. 



group, it is clear that it belongs to a period subsequent to the deposition of this group. There 

 being no newer rock in this region than the slate referred to, it is impossible to bring the date 

 of its formation to a later period. 



From what has been said of the characters of igneous rocks in the preceding chapters, it 

 will be unnecessary to state here the reasons for adopting the opinion that this mass of por- 

 phyry is an igneous injection. The facts which I have had occasion to state, fully confirm 

 this view ; and yet there is one which, though it has been alluded to, ought to receive a mo- 

 ment's attention : it is, that though the mass must be considered as igneous in its origin, 

 still it must have been in a state quite different from the ordinary rocks of this class, inasmuch 

 as there is no change produced in the texture, structure, or organic remains of the slate 

 with which it is in immediate contact. Specimens of graptolites, and remains of the 

 Trinuclcus tessellatus, were found unchanged, and almost in immediate contact with the por- 

 phyry. This fact then seems to favor the conclusion that the mass was in a state of thick 

 paste, and like the matter sometimes ejected from volcanoes in a state of mud, but whose tem- 

 perature was comparatively low, or not sufficiently high to act on the rocks with which it came 

 in contact. 



Varieties, etc. — There are only three or four varieties of this rock : the reddish brown, 

 which is the most common ; the pale-leek green ; the jaspery, which is a hard compact stone, 

 with a fracture somewhat conchoidal ; and the dendritic. The last differs from the ordinary 

 infiltrations, so frequent upon the outside of the layers of rocks : it is exhibited in the interior 

 of the irregular columns. The dendritic matter has penetrated nearly to equal depths on all 

 its sides, forming a beautiful arrangement : it extends to the depth of two or three inches in 

 some pieces ; while in others, only to a half or one-fourth of an inch. 



To give a rationale of the movement which produced the forms and figures within the 

 stone, it seems probable that the whole rock was immersed at one time in a watery medium, 

 containing manganese or the oxide of iron in solution or suspension, which penetrated not 

 only between the columns, but was forced into the columns themselves by a movement 

 called endosmose. Each separate piece has its own figure complete, penetrating into the in- 

 terior according to the time during which it was immersed in the fluid, or in proportion to 

 the facility with which the stone absorbed the metallic oxide. 



This rock, though not important in an economical point of view, is still interesting on account 

 of its relation ; and besides, it is so distinct from all other rocks in the vicinity, and confined 

 to a small territory, that its boulders become very good guides for the determination of the 

 direction of the currents of water which have swept over this section of country. In this 

 respect, it compares with those of the hypersthcne rock, which may be so readily distinguished 

 from gneiss or granite. I have often seen the porphyry in loose pieces in the soil fifteen or 

 twenty miles south of Cannon's Point, and in a direction nearly south of this locality ; and if 

 I may rely upon memory, it is not found far to the east or west of a north and south line. 



This mass has evidently been broken up quite extensively ; as its fragments, in a rounded 

 state, line the shore for long distances, particularly towards Split Rock, where immense quan- 



