BASAL GNEISSES OF THE HIGHLANDS 375 



nearer the margin. One of these follows closely the northern 

 base of Storm King mountain and Breakneck ridge. Where it 

 is best exposed, a mile southwest of Cornwall Station, the walls 

 show a fault plane dipping steeply to the southeast with the 

 granites of Storm King overriding the Hudson River slates. 

 This overthrust, therefore, represents a displacement of prob- 

 ably 2000 feet and perhaps more. The overthrust tendency from 

 the southeast is apparent at many other places, and it serves to 

 create some most abnormal relations throughout the Cambro- 

 Siluric area lying to the north where overthrusts of great com- 

 plication are the rule, such as that at New Hamburg or at 

 Cronomer hill, north of Newburgh. 



The other notable fault of even greater significance occurs 

 on the south side of the Highlands proper and the escarpment 

 along this line marks a physiographic and stratigraphic boun- 

 dary for many miles. This fault line follows the west side of 

 Peekskill creek to the Hudson river, crosses to Tompkins Cove, 

 and then passes to the southwestward across the New York 

 State boundary into New Jersey. It sharply limits the High- 

 lands Precambric and its displacement established an escarpment 

 against which the Triassic sediments were laid down and which 

 yet marks their interior limits. Present conditions of the strata 

 preserved along this line indicate two separate movements : first, 

 block faulting and tilting by which the south wall was dropped 

 probably 2000 feet or more, carrying down into the trough thus 

 formed all the overlying Cambro-Siluric formations that at that 

 time covered the Highlands ; later a thrust from southeast closely 

 folding the sediments entrapped in this trough and in places 

 thrusting the gneisses upon them. The net result is a preser- 

 vation of representatives of the later group of formations (Cam- 

 bro-Siluric) along this fault line. This is especially successful 

 on the margin of the down faulted block, so that, in Peekskill 

 Creek valley and the next small valley to the southeast, and 

 from Tompkins Cove southwestward for some distance, these 

 formations may be seen. But because of the somewhat similar 

 succession of members and character, and because of the over- 

 sight of this profound structural break, the identification of the 

 formations of this district has been confused with the older 

 group. It is the writer's belief that this allowance made for 

 the occurrence of the seemingly abnormal strata in the southern 

 Highlands permits reasonable explanations for all occurrences 



