Smyth — Crystalline Bocks of St. Lawrence County. 



41)7 



The gneisses constitute a complex series of rocks, differing somewhal 

 in age, and largely, if not almost wholly, of igneous origin ; parts <>f this 

 series are clearly younger than the limestones, and while other parts may be 

 older than the latter formation, there is nothing as yet to prove that such is 

 the case. An exception to the latter statement is probably afforded by 

 certain laminated gneisses of limited extent, which appeal- to underlie the 

 limestone, perhaps marking the base of the series. This relationship, is, 

 however, inferred rather than proved. 



If these inferences are correct,. ■ the task of mapping involves the 

 tracing of the limestones together with such minor portions of gneiss as 

 belong with them ; and, so far as possible, the differentiation of the gneisses 

 upon a chronological and petrological basis. As regards this latter part of tin- 

 work, it is questionable whether, in many cases, any very sharp distinctions 

 can be made out, or definite lines of demarkation laid down, as the nature of 

 the rocks seems to be such as to preclude anything but rather broad 

 generalizations. It may be, however, that under careful study the present 

 difficulties will become less, and greater accuracy be attained than now 

 seems probable. Much that is now obscure will, without doubt, become 

 entirely clear when the whole area is studied with all possible care and detail. 

 Such a general reconnaissance as has been made serves only to prepare the 

 way for this more detailed study, indicating the problems involved, and 

 sometimes pointing to their probable solutions. The region is certainly 

 deserving of such careful examination as will lead to the preparation of an 

 accurate geologic map. 

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