110 pp. REPOKT OF PROGRESS. FONTAINE & WHITE, 



several sources and give preponderating weight to what 

 would, if unaffected by them, have slight value. 



Having thus established the evidence of one class of or- 

 ganisms, we must combine it with that of any other class 

 afforded by the strata, and the general facies of the entire 

 life only can be used in determining the age. But the 

 evidence from this source must not stand alone, if we can 

 supplement it with that derived elsewhere, we must search 

 all possible sources. 



A source from which we may often derive evidence of 

 great value is the stratigraphy, and especially the lithology. 

 Many geologists unduly depreciate the value of the latter. 

 It is easy to see that where the strata have such a character, 

 that they could only be formed under special conditions, 

 they must have a certain value as evidence, especially when 

 this is combined with the diminution, or disappearance of 

 beds characteristic of a certain formation. Thus in ascend- 

 ing from a known carboniferous horizon, to superimposed 

 formations, if we find the coal abundant in the lower beds, 

 and disappearing in the upper, while great masses of lime- 

 stone and fine grained red shales come in, surely this would 

 be weighty evidence to show that carboniferous conditions 

 had changed to Permian. If the life of the period is very 

 scantily represented by fossil forms, cases might occur 

 where we would be called upon to determine the relative 

 value of the two, and we would find the evidence of Lith- 

 ology of superior weight. It is not impossible to find cases 

 where the evidence of Lithology resembles in character 

 and degree that of fossils. Certain strata may have such 

 a peculiar character, that when their eroded fragments enter 

 into the composition of later formed brecciae, or conglomer- 

 ates, they may be recognized with certainty a hundred 

 miles and more from the parent source. This is actually the 

 case with certain conglomerates of the eastern portion of 

 Virginia, which are of Potsdam quartzites. 



We may also employ the evidence to be derived from the 

 ''Breaks" and physical changes found to occur. If two 

 formations are separated by strata giving evidence of a 

 change in the prevailing conditions sufficient to cut off or 



