500 



THE DEVONIAN PERIOD. 



This junction is of great interest, as showing the gradual alteration 

 of slaty beds holding fossils into gneissose rock with garnets, within 

 the distance in some places of a few hundred feet. It is observable 

 also, that while the gneiss graduates into the slate it does not pass 

 imperceptibly into the granite, but presents a distinct line of separation, 

 marking the limit of the Plutonic and Metamorphic rocks, and indi- 

 cating that the granite was truly a heated mass intruded among the 

 aqueous deposits (Fig. 177). Farther, as the granite is itself of 



Fig. 177. — Junction of Granite and Devonian Slate, Nictaux. 



(a) Granite. (b) Slate with gneissose character, in fragments imbedded in the granite. 



Devonian age, we learn that no great interval of geological time 

 elapsed between the deposition and the metamorphism of the beds. 

 Again, as the granite cannot be a superficial or surface rock, there 

 must have been a mass of upper Devonian rocks swept away by 

 denudation to expose the beds as at present. Lastly, though the beds 

 are inclined at high angles, they run against the granite in their line 

 of strike in such a manner as to s»how that it cuts quietly through 

 them, without any great evidence of mechanical disturbance in con- 

 nexion with its eruption, and it would appear that the general direction 

 of dip is toward the granitic mass, as if the Devonian and Upper 

 Silurian beds had sunk into a caldron of molten granite. Further 

 exploration of the country southward of Nictaux will be necessary 

 before we understand in detail the relation of the Devonian rocks to 

 the great masses of granite which appear in this direction. 



Westward of the Nictaux River, the granite abruptly crosses the 

 line of strike of the slates, and extends quite to their northern border, 

 cutting them off in the manner of a huge dike from their continua- 

 tion about ten miles further westward. The beds of slate in running 

 against this great dike of granite, change in strike from south-west to 



