walcott. ] CORRELATION. 405 



On the map of Adams County, based upon the results of geological 

 work of Dr. Persifor Frazer, the quartzite resting upon Azoic is iden- 

 tified as Potsdam, with a question mark. 



Safford. — In his first report upon the geology of Tennessee, Prof. J. 

 M. Safford describes the method of the grouping of the strata into for- 

 mations. He says: 1 



We often meet with a series of adjacent strata .similar iu many respect*, ho much 

 so that they can conveniently be thrown together iu a single group or formation, as 

 such a series is often termed. 



He says further : 



For example, the sandstone, states, and coal which form the upper part of the Cum- 

 berland Table-land, are grouped in & formation, called the Coal Measures, with the fol- 

 lowing among other common characters : First, eoal is found at intervals throughout 

 the series; secondly, there is very little limestone or calcareous matter in the series ; 

 thirdly, the strata are parallel, and appear to have been formed in succession, under 

 similar circumstances; fourthly, the same fossils, such as different species of petri- 

 fied shells, leaves, branches, and trunks of trees, etc., occur imbedded in the rocks 

 throughout the length and breadth of the series. Characters similar to these unite 

 the strata of all the formations. 



The character last mentioned we must refer to more particularly, on account of its 

 great importance in designating with precision the group to which local and isolated 

 beds of rock belong. 



In defining the use of fossils it is stated that each formation has, in 

 great part, its own fossils. 



Most of those found in one do not occur in any other. Upon this fact depends their 

 great utility. They furnish, when known well enough to be recognized, unmistak- 

 able evidence of the geological position, and hence the general character of the for- 

 mation in which they are found. 



From these observations of Prof. Safford we postulate the principles 

 of correlation which he used, as follows: 



I. Stratigraphic position. 

 II. The occurrence of similar fossils. 

 III. Lithologic characters. 



CANADA. 



The New York series was traced into Canada by Sir Wm. E. Logan 

 and his associates and correlations made by direct stratigraphic connec- 

 tion, and iu the absence of that, paleontoiogic data, stratigraphic rela- 

 tions, and lithologic characters. 



Logan. — In speaking of the stratigraphic succession of the New York 

 and Canadian sections Sir W. E. Logan makes the following reference 

 to the fossiliferous limestone above the Calciferous sandrock, or the 

 Trenton limestone : 



The lowest of the fossiliferous strata is a sandstone of variable quality, more purely 

 siliceous towards the bottom, and calciferous towards the top, which gives support 

 to a thick and remarkably persistent deposit of limestone, strongly distinguished by 

 its organic remains. This limestone thus becomes an admirable means of tracing 



1 A geological reconnaissance of Tennessee ; first biennial report, Nashville, 1856, pp. 130, 131. 



