I094 PRINCIPLES OF STRATIGRAPHY 



Mechanical Deformation of Fossils. 



Wherever rocks containing fossils have been under pressure, 

 or have, through other means, suffered mechanical disturbances, 

 the fossils commonly show a more or less pronounced deformation. 

 Two types of deformation may be considered: (i) that due to 

 the normal desiccation and consequent shrinking of the rocks in 

 otherwise undisturbed regions, and (2) that due to orogenic dis- 

 turbances. The first type is especially marked in shales, and is due 

 to the vertical pressure exerted by the overlying rock and the verti- 

 cal shrinking of the shales upon the loss of water. Shells of 

 brachiopods and pelecypods are commonly flattened out, while gas- 

 tropods, cephalopods and trilobites are most frequently distorted by 

 this vertical pressure. The amount of compression can sometimes be 

 estimated by noting the sagging of the strata on either side of a con- 

 cretion enclosed by them. Again, it may be estimated from a com- 

 parison of the compressed shell with uncompressed examples from 

 the same formation, but preserved in limestone bands or lenses. The 

 latter have suffered little or no vertical compression on account of 

 the fact that the component grains of the rock were already as firmly 

 packed when the rock solidified as they were ever likely to be. 



The second group begins with the deformations due to horizon- 

 tal slipping within a stratum, owing to the pressure of a superin- 

 cumbent mass. Under such circumstances slickensides are fre- 

 quently produced within a given formation, and fossils may readily 

 be affected by such movements. Lateral compression of the strata, 

 either slight or sufficient to produce foldings, will distort the fossils 

 embedded in them and not infrequently alter their form, so that 

 they are no longer recognizable. A brachiopod, for example, by 

 compression may assume the outline of a pelecypod, and may readily 

 be mistaken for one. When, through strong compression, cleavage 

 is induced in a given stratum, the fossils of that bed may become 

 largely or entirely destroyed. The same is true if metamorphism 

 affects the strata, though occasionally, as in the Palaeozoic of Scan- 

 dinavia, fossils are found in schists and other metamorphic rocks. 



Index Fossils. 



Fossils which serve to indicate definite geological horizons are 

 called Index Fossils (German, Leitfossilien). The best index 

 fossils for marine formations are furnished by invertebrates, though 



