Smyth — Crystalline Hocks of St. Lawrence County. 



493 



difficult to secure. As already reiterated, there is also no evidence that any 

 considerable part of the gneiss is of the same age as the limestone; while 

 many facts are opposed to this supposition. As to the third supposition, that 

 the gneiss is, at least in part, younger than the limestone, there is positive 

 evidence, and of such a nature as to give, at the same time, abundant proof as 

 to the origin of these portions of the gneiss. 



An example of this kind in Pitcairn and Diana has been previously 

 described, and need be only briefly referred to here. A large area of plutonic 

 rock, in some parts gabbroitic, in others syenitic or granitic, forms the southern 

 boundary of the limestone belt between Natural Bridge and Harrisville. The 

 igneous nature of the rock is shown not only by its composition and 

 structure, but also by the fact that it is clearly intrusive in the limestone, 

 cutting it along a very irregular line with the production of well defined 

 contact zones containing fl variety of minerals. 



The plutonic rock is often slightly gneissoid and in thin sections generally 

 shows more or less cataclastic structure. Passing south and west from its 

 boundaries these features rapidly increase and the rock becomes a reddish 

 gneiss of medium grain. In itself there is no important feature to distinguish 

 this gneiss from other gneisses, its only exceptional feature being its evident 

 identity with the large area of plutonic rock intrusive in the limestone. Here 

 then, is an important area of gneiss, for which the two chief problems are 

 solved ; it is a modified plutonic rock, and is younger than, and intrusive in 

 the limestone. No other instance has been found where the phenomena are 

 exhibited on so extensive a scale, but facts similar in kind have been noted 

 at several points. 



In the paper just referred to, it was stated that the southern border of 

 the gneiss area between the Edwards and Pitcairn limestone belts, was com- 

 posed chiefly of rocks undoubtedly igneous and intrusive in the limestone. 

 An effort has been made to ascertain the relations on the northern edge, and 

 in the one contact found between the two formations similar phenomena w ere 

 observed. The locality is about three-fourths of a mile south of the village of 

 Edwards. Here the gneiss is of the ordinary, rather fine type, which is so 

 widespread. Its relation to the limestone is clearly show n, and it is seen to 

 break through the latter formation with a most irregular irruptive contact. 

 As in the case of the Pitcairn rocks, there is pronounced contact-metamorphism 

 resulting in a formation of zones composed largely of coccolite, together with 

 feldspar, scapolite, etc. This locality differs from the first described, in that 

 it is the gneiss itself, and not the parent plutonic rock, which is in contact with 



