Smyth — Crystalline Rocks ok St. Lawrence County. 



485 



to the broader problems, and of these, one is certainly the distribution of the 

 limestones and their relation to the other rocks. 



The mere facts of distribution have been given above, their bearing upon 

 the question of relation to other rocks will be considered in dealing with the 

 relations of the limestones to the gneisses. This question requires, however, 

 for its discussion, a previous consideration of the probable origin of the 

 gneisses. 



Origin of the Gneisses. From the foregoing it is evident that the 

 gneisses afford the chief problem of the region. This problem is twofold, 

 involving both the origin of the gneisses and their relation to the limestones. 

 While these two questions are intimately connected, and facts bearing on one 

 are often equally important in their relation to the other, it may be more 

 convenient to consider them separately. 



In the outset it should be stated that neither question is regarded as 

 finally settled. It is often very difficult to procure data which bear definitely 

 upon these questions, and as the rocks involved cover a great area, it is 

 dangerous to generalize upon a basis of facts gathered in a small portion of 

 the area. This difficulty has been somewhat reduced by the agreement of 

 phenomena occurring at widely separated points, but it is, nevertheless, a real 

 difficult)". In man}' cases, perhaps in most, no direct evidence is at hand, and 

 it is necessary to reach conclusions by analogy. For these reasons, particu- 

 larly on account of the limited amount of study as compared with the 

 extent of the region, it is not deemed wise to state any very broad final 

 conclusions, but merely to present such facts as seem to be of importance 

 in their bearing upon the major problems of the region. What this bearing- 

 is, and to what conclusions the facts in hand seem to lead, must, of course, 

 be stated; but the endeavor will be made neither to exaggerate the 

 importance of the data, nor to deal with them dogmatically. 



Without going into the history of opinion on the subject, it may be 

 stated that the gneisses have generally been regarded as metamorphosed 

 sediments, such parallel structures as they possess being considered as result- 

 ing from original bedding. 



The term gneiss is here used in a very broad sense, embracing rocks 

 having a wide variation in structure and composition. The chief value of the 

 term, when used without any qualifying word, lies in its breadth, and this 

 value largely disappears if the term is restricted t<> a rock having the compo- 

 sition of granite. Such a broad term is essential in the present discussion, 

 not only on account of the lack of detailed investigation, but chiefly because 



