5^ NEW YORK STATE MUSEUM 



3 Major structural features 



In addition to the simpler structural characters of the strata, 

 already sufficiently emphasized in the individual descriptions, there 

 are numerous others of more general relation whose value and in- 

 fluence it is necessary to consider in many of the practical problems. 

 Those of most importance are the unconformities, folds and faults. 

 They are directly related to continental elevation and subsidence, 

 to mountain forming movements and denudation processes, to meta- 

 morphism and to igneous intrusion. 



a Sedimentation structures. In the younger strata the prin- 

 cipal structures are those of bedding, stratification, conformable 

 succession, etc., characteristic of all sediments of such variety of 

 type. These are prominent in the older groups of formations down 

 to the crystallines, but the earlier Paleozoics are also affected so 

 profoundly by folding and faulting that attention is more concerned 

 with these induced or secondary structures. 



h Unconformities. Time breaks, with more or less disturb- 

 ance of strata and accompanied by erosion, are numerous. 



(i) That between the glacial drift and the rock floor is the most 

 profound. It causes the glacial drift to lie in contact with every 

 formation of the region from the oldest gneisses of the Grenville 

 series of the Highlands to the traces of Miocene beds of Long 

 Island. 



(2) The interval between the Pliocene and the Upper Creta- 

 ceous beds is more obscure and hardly reaches the importance of 

 an unconformity. It is probably more nearly of the value of a 

 disconformity or of an overlap, and the very limited development 

 of the overlying beds in the region gives little chance for determin- 

 ing relations in much detail. 



(3) The overlap and unconformity between the Cretaceous and 

 Triassic. A condition determinable only on the New Jersey side 

 of the Hudson river. 



(4) The unconformity between the Triassic and underlying 

 formations of difl'erent ages. An interval representing mountain 

 development and extensive erosion, in which the chief movement 

 probably belongs to the close of Paleozoic time and includes the 

 Appalachian folding. 



(5) Unconformity between Siluric and the Ordovicic strata. An 

 interval representing mountain development, folding and erosion, 

 in which the movement known as the Green Mountain folding took 

 place. 



