REPORT OF THE STATE PALEONTOLOGIST 1902 1209 



STRUCTURAL FEATURES 



The cross sections on plates 3, 4 and 5 portray the large tectonic 

 features of the Vlightberg and North hill. No attempt has 

 been made to illustrate in these sections the smaller folds and 

 faults which are so numerous in all the quarries and mines. 

 Illustration of these details is reserved for a future paper. 



The term " vein " is employed in the Rondout region in a sense 

 similar to that in which it is used in the coal regions, namely 

 for a single layer or for a composite series of layers of sedi- 

 mentary rock quarried for economic purposes. It has no rela- 

 tion to and should not be confounded with veins of metallic 

 ore deposits. 



The compass readings here given have all been reduced to true 

 meridian on the basis of 9°30' west declination for Rondout. 



The unconformity between the Wilbur limestone and the Champlainic 



sandstone 



The Wilbur limestone and the underlying Champlainic sand- 

 stone and shale are sharply unconformable at all points in 

 this vicinity where the contact of the two formations can be 

 observed. It can, however, be seen only along the east side of 

 the Vlightberg and in the underground Vlightberg mines. The 

 nature of this unconformity has been well described by Davis j 1 

 and we can add only a few details to his description. The sur- 

 face of the tilted sandstones at the unconformity, a surface 

 which represents a very ancient plane (probably Eosiluric) of 

 subaerial degradation, is much more uneven at some points 

 than at others. At the locality marked U near the south end 

 of the Vlightberg, where the unconformity was studied by 

 Davis, and where our photograph [pi. 7] was taken to show the 

 same view illustrated by Davis's sketch [1883. p. 392], the 

 eroded edges of the sandstone form a very irregular surface 

 with hollows 6 to 24 inches deep. These hollows are in the 

 nature of grooves parallel to the bedding planes of the rock, 

 and the largest grooves coincide with the most prominent de- 

 position planes, the latter having furnished lines of least resist- 

 ance for the eroding agent. The hollows are filled with the 



1883. p. 392-93 ; and 1890 . p. 355. 



