J _i_ A^ A ll_"X . ■ - I T 



92 NEW YORK STATE MUSEUM 



Here the strata are nearly or quite concordant though there 

 may occur a discordant line of contact between the two as 

 shown in figure 3 b. This latter type of unconformity is not 



readily recognizable, 

 and in fact would not 

 be noticed except for 



Fig. 3a Stratigraphic unconformity , . ,. , , i , , 



the fact that a number 

 of formations are missing between the strata in question. In the 

 past very many such stratic unconformities have been overlooked, 

 with the result that the geologic history of the region in which 

 they occur has been misinterpreted.- 



The time intervals represented by the two unconformities 

 indicated in the table are considerable. The one at the base 

 of the series is by far the greatest, representing not only the 

 Middle and Lower Cambric but probably also a considerable por- 

 tion of late Algonkian time. The stratic break between the 

 Ohamplainic and Siluric represents in this region the time 

 interval during which the Lower and Middle Siluric strata were 

 deposited in other localities. The details of this uncomformity 

 will be discussed under the chapter dealing with the Ohamplainic 

 and Siluric strata. 



Another principle which is of 

 great importance in the inter- 

 pretation of the geologic historj^ 

 of this region, is that of j^^'o- 



gressive overlap of the strata and the attendant change in the 

 character of the strata themselves. This principle, while gener- 

 ally recognized in a tacit manner, is too often overlooked where a 

 direct application should be made. A brief discussion and illus- 

 tration of the principle may therefore be given, specific illustra- 

 tions from this region being deferred till the strata involved are 

 discussed. 



It is a well known fact that along a normal seashore where 

 there is a difference in the texture of the detritus, the coarsast 



Fig. 3t) Stratigraphic unconformity 



1 The author has recently proposed the term discomformity for t]ii.s tyj); 

 of break in the stratigraphic series [Science, n. s. 22:534]. 



