114 LITHOLOGICAL GEOLOGY. 



ones of the bottom (as c with d). In one instance a mistake was 

 thus made with respect to the age of a human relic, which a 

 little care might have avoided. In other 

 cases, caverns in rocks have been filled Fi S- 114 - 



through openings from above, and the ^L _ « 

 same kind of mistake made. When the hj^j^j 

 continuity can be established, the evi- | ^ 



dence may sometimes lead to important ,Jpllllil|llp^7rr7>. 

 results. For example, it may be found fe^'^^^g.^^ ^^^ 

 that a coal-bed followed for some miles I " ' '•'•'•'— - ' ^^l| 



to one side or the other is continuous with 



a clay shale, and both are actually one layer ; that a sandstone is 

 one with a limestone a few miles off; that an earthy limestone full 

 of fossils is identical with a layer of white crystalline marble in a 

 neighboring district ; or that a fossiliferous shale of one region is 

 the same stratum with the mica schist of another. 



Precaution 3<f. — Note whether the strata overlie one another con- 

 formably or not. 



Precaution 4th. — When one bed overlies another conformably, it 

 does not follow necessarily that they belong to consecutive periods. 

 The Tertiary beds may rest conformably on any stratum from the 

 Cretaceous to the Silurian. A range of conformability so conti- 

 nuous is not, however, usual ; for disturbances of the strata have 

 been so common in past time that the later rocks are seldom con- 

 formable to the older. Among rocks comparatively near in age, 

 however, it is common to find strata wanting, without any break 

 in the conformability. 



The criterion mentioned, unless connected with others, gives no 

 aid in comparing the rocks of distant or disconnected regions. For 

 this purpose other means must be employed. 



124. (2.) Color, texture, and mineral composition. — This test may be 

 used to advantage within limited districts, yet only with caution. 

 There were at one time in geology an "old red sandstone" and a 

 "new red sandstone," and whenever a red sandstone was found it 

 was referred at once to one or the other. But now it is well under- 

 stood that the color is of little consequence, except within a small 

 geographical range. The same general remark holds with reference 

 to mineral composition. 



One inference from the constitution of a stratum is safe ; that is, 

 that the stratum is more recent than the rock from which its mate- 

 rial was derived. Hence an imbedded fragment of some known 

 rock may afford important evidence with regard to the age of the 

 •containing stratum. 



