496 



Report of the State Geologist. 



doubtful. But, as bearing upon this, it should be said that it is a very 

 general rule in the region that intrusions of limited extent tend t<> assume a 

 form decidedly elongated in the direction of the strike, and, as a result, often 

 have the appearance of being interbedded, even where the facts clearly show 

 their intrusive nature. 



The relations between the gneiss and the granite intrusive in the lime 

 stone must be considered in connection with the question under discussion. 

 In the previous report it was stated that the granite often becomes very 

 gneissoid, so much so that in some cases it can be distinguished only bv its 

 relation to the limestone. On this account no effort was made to separate 

 granite from gneiss in the large areas of the latter. As the gneiss has been 

 more carefully studied, the fact has appeared that much of it is quite as 

 massive as the granite, and decidedly more so than the gneissoid phases of 

 the latter. No petrographic distinction can be made between the massive 

 gneisses and the granite, a fact of much importance in the present connection, 

 for it at once leads, to the inference that the granites may be offshoots of 

 the gneiss, affording another indication of the origin and age of the latter. 

 The existence of granitic areas in the gneiss was not doubted in the previous 

 report, but it was felt that they might be of minor extent and importance, and 

 so it was stated that the explanation of the great gneissic areas as intrusive 

 must be employed with caution. 



In addition to the facts enumerated above, many cases have been observed 

 where the structural relations of the gneiss and limestone are highly indicative 

 of the intrusion of the former, although the outcrops do not suffice to prove 

 conclusively such a relation. 



The data presented in the foregoing necessitate the following conclusions : 

 All parts of the gneiss are not of the same age; some portions, of large 

 extent, are of igneous origin; of these igneous gneisses part, at least, are 

 younger than the limestone. These conclusions are merely the statement of 

 what is clearly and definitely proved ; but, with the facts in hand, some more 

 general inferences seem to be entirely justified. Indeed, in the case of 

 extensive areas of the gneisses, positive evidence as to their origin and relation 

 to other locks will probably never be found, and the problems can be 

 solved only by inference and analogy. Keeping in view what has been 

 stated in regard to the rocks, their uniformity, gradual transitions, massive 

 phases, inclusions, irruptive contacts, etc., scattered over widely separated 

 ureas, it would seem that the following inferences may be draw n as, at least, 

 extremely probable : 



