1 130 PRINCIPLES OF STRATIGRAPHY 



each has been ascertained. The general law of superposition is: 

 that, of any tivo strata of sedimentary rocks, the one zvhich was 

 originally the lozver is also the older. This does not, of course, 

 apply to intruded igneous rock, for a much later sheet of intrusive 

 material may find its way between strata very much older and 

 so be followed by strata older than itself. In exceptional cases, 

 too, sedimentary rocks may not follow this rule, as in the case of 

 deposits in caverns cut out of older rocks. Here strata may 

 actually form beneath the surface of the lithosphere and hence 

 below an older stratum. 



Of course, in regions of faulting and strong folding, the order 

 of the strata may be reversed, so that it becomes necessary, first 

 of all, to demonstrate the original position of the formations. 



In the ascertainment of the superposition of strata of a given 

 region great care must be taken to note the existence of strati- 

 graphic breaks. Disconformities of strata are often difficult to 

 recognize, but unless ascertained are sure to introduce an element of 

 error into the geologic columns of the region. Abrupt changes 

 in sedimentation are a useful guide in the location of such discon- 

 formities and, in fact, where such sudden changes occur it may 

 be taken as an indication of the possibility of the existence of such 

 a hiatus, though this alone is not sufficient proof of its existence. 

 A good example of a great hiatus -indicated only by an abrupt 

 change in the character of the rock is found in the case of the con- 

 tact of the Black Chattanooga shale with the gray Rock wood clays 

 in eastern Tennessee. Here there seems to be, at times, no other 

 indication, than this sudden change in character, of the absence, 

 between these two formations, of more than the entire Devonic 

 system of strata. Indication of erosion surfaces, and the inclusion 

 of the fragments of the lower in the upper beds, commonly charac- 

 terize the disconformity, but give no clue as to the magnitude of 

 the hiatus. The change from one lithic unit to another may be 

 abrupt, witliout necessarily indicating a disconformity. In such 

 cases, generally, there is some alternation of beds of the two series 

 before the complete disappearance of the lower series.. Thus the 

 Black Shale at the top becomes intercalated with thin bands of 

 the overlying formation, and itself occurs at intervals in the form of 

 thin layers for some time after the extensive development of the 

 overlying series. ^ 



Correlation by superposition, however, is a method fraught 

 with grave dangers. Thus a succession of formations from sand- 

 stones to shales and limestones in one part of a province is not 

 necessarily the same as a similar series in another part of the same 



