492 



Report of the State Geologist. 



embayrnents in the blocks are /filled with gneiss showing this structure 

 parallel to the sides. 



To account for the masses as anything but inclusions in an igneous rock, 

 is even more difficult than in the case of the bands. All of the phenomena point 

 to this origin, w hile they present great difficulties in the way of any other 

 explanation. It would seem, then, that at many w idely separated points the 

 gneiss contains masses that can hardly be explained as anything other than 

 inclusions of some older formation taken up by the gneiss when it was in a 

 molten state. At some points the evidence is much more decisive than 

 at others, and while in the more obscure occurrences other explanations 

 might suffice, in the typical cases the one offered seems to be demanded. 



From the foregoing it is apparent that the gneiss affords considerable 

 internal evidence bearing upon the question of its origin, and all of the 

 evidence points in one direction. To sum up briefly: there is the negative 

 evidence of the absence of all structures pointing to sedimentary origin ; 

 there is the uniformity of composition and structure over wide areas, with 

 changes by gradual transition ; there is the common occurrence of massive 

 coves, in every way identical with plutonic rocks, and the presence of 

 structures in the gneiss that would result from the application of pressure 

 to such rocks; there is the existence of irruptive contacts between the 

 abundant light-colored gneiss and the less common, and older, dark gneiss, 

 together with the widespread instances of inclusions of the dark gneiss 

 in the light. 



While no one of these lines of evidence might be regarded as conclusive, 

 it is believed that, leading as they all do in one direction, their cumulative 

 force is great. 



There still remains the external evidence bearing upon the problem; 

 and as this involves the relationship between the gneiss and the lime- 

 stone, this latter problem must of necessity be considered. 



The absence of any sufficient reason for regarding the gneiss and the 

 limestone as portions of one series has been already pointed out, If 

 distinct, the gneiss is either older or younger ; and in the former case the 

 relations of the two formations would shed no light upon the origin of 

 tin- gneiss, while in the latter they might afford important evidence. 



No facts are at hand w hich prove conclusively that any part of the gneiss 

 belongs to a series older than the limestone, but this mere absence of evidence 

 can not be regarded as excluding the possibility of the presence of such an 

 older gneiss series, for, in the nature of the case, positive evidence is 



