WALC0TT.1 CORRELATION. 429 



Lower Cambrian quartzite of the western slopes of the Green Moun- 

 tains, the Upper Cambrian Potsdam sandstone of New York, the pre. 

 Cambrian Sioux quartz i tea of Minnesota, the pre-Cambrian Baraboo 

 quartzite of Wisconsin, and the pre-Cambrian quartzite of the Kewee- 

 naw an series of Lake Superior. 



Lithologic character is often of great value within limited areas and 

 where the formation can be practically traced from place to place. As 

 this can rarely be done, it is best to seek other data and to combine 

 with the lithologic, the stratigraphic and paleontologic evidence, if it is 

 possible to obtain any. 



IV. UNCONFORMITY. 



Of all the data of correlation that of unconformity of the formation 

 correlated, with its superjacent or subjacent formation, would appear at 

 first sight to be the most uureliable. In most instances it is, but where 

 combined with other data it has a certain value. We find it used in the 

 earliest correlations made of the Potsdam sandstone of New York, with 

 the basal sandstone all along the Appalachian Range south through 

 New Jersey, Pennsylvania, Maryland, Virginia, Tennessee, Georgia, and 

 Alabama. It is also used iu the correlations made between the Pots- 

 dam sandstone of New York and the saudstoue of the south shore of 

 Lake Superior, and the fossiliferous sandstones of Wisconsin, Iowa, 

 and Minnesota, that were correlated with the Potsdam sandstone by 

 Messrs. Owen, Hall, and later observers. In fact, it is implied in 

 nearly all the references of strata to the Potsdam horizon in other local- 

 ties than that of the typical area about the Adirondack Mountains of 

 New York. It was not, however, until the appearance of Prof. K. D, liv- 

 ing's essay on the classification of the earlier Cambrian and pre-Cam- 

 briau formations 1 that full significance was given to the value of un- 

 conformities in correlation. 



The problem is presented by Dr. Irving as follows: 



(1) To determine upon the grander divisions (groups) to he ma de m classifying the 

 rocks independently of their relations to the general geological column ; (2) to ex- 

 tend these divisions to other portions of the same geological basins; and (3) to cor- 

 relate these divisions with those of different and distant geological regions. This is 

 the problem which presents itself in the Lake Superior region, at the base r»f the 

 Grand Canon of the Colorado, in central Texas, Newfoundland, and in other por- 

 tions of North America. 



In any attempt to solve such a problem we can make use of one or more of three 

 kinds of characteristics in the formations involved, viz : (1; their paleontologies! 

 characteristics, (2) their lithological characteristics, and (13) their mutual structural 

 relations. 3 



Under the third title, "their mutual structural relations, " he con- 

 siders unconformity as a basis for classification 3 discussing the general 

 nature and significance of conformities and the distinguishing char- 

 acters of true unconformities. A description is given of the Potsdam- 



1 Seventh Ann. Eep. U. S. Geol. Snrv., 1885-'86, 1888. 

 2 Op. cit., p. 371. ^Op. cit, p. 390. 



