GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK IO7 



that of typical regional metamcr'TJiism that they can not be distin- 

 guished. If, for example, this latter possibility will hold, then it 

 may be that the Grenville did not pass through a period of regional 

 metamorphism before its igneous history began. But if, on the 

 contrary, igneous invasion alone will not accomplish this type of 

 transformation, then it is necessary to make allowance for a greater 

 period of time, and a whole epoch of dynamic and earlier history 

 than would otherwise be necessary to assume. 



The writers of this bulletin, appreciating fully the probability 

 that igneous influences are capable of accomplishing profound 

 results, and that in this district igneous phenomena and effects are 

 much more prominent than dynamic effects, still hold that certain 

 types of metamorphic phenomena are not exactly duplicated by 

 igneous methods. We believe that strong schistose development 

 of a series of rocks, with normal distribution and relation of minor 

 structures, indicates dynamic metamorphism as the fundamental 

 cause, 



A closely folded series ot strata with strongly schistose structure 

 ought to furnish the conditions favorable to just the sort of effects 

 produced by the later igneous history more successfully than rock 

 in any other condition, and especially better than simple sediments 

 essentially undisturbed. We regard it as practically certain, there- 

 fore, that the Grenville passed through a period of regional metamor- 

 phism and folding previous to its chief igneous history. 



This period doubtless belongs to the Huronian also and it may 

 very well be that a very great length of time even for Precambrian 

 epochs has been covered. If the Manhattan-Inwood series is 

 included with the Grenville it is all the more certain that dynamic 

 metamorphism belongs to its earlier history, but the uncertainties of 

 this series make it valueless as a point of argument on this question. 



To accomplish regional metamorphism of the sort suggested would 

 require that these strata lie under a load of other sediments, and 

 thus a great sedimentation history preceding metamorphism is 

 indicated. Metamorphism was, we believe, caused by folding which 

 transformed the original sediments into foliated types ; in short, a 

 great variety of schists, phyllites, quartzites, graywackes and lime- 

 stones. The structural character assumed in that period both in a 

 large and small way has guided or at least has influenced all subse- 

 quent structural development. This folding was followed or per- 

 haps accompanied by erosion, but whether the area was again the 

 seat of sedimentation during the Precambrian is not indicated. 



