INTRODUCTION 



21 



ripples, cracks in the mud due to drying, imprints of rain drops, 

 flows of mud due to an excess of water, and the marks showing 

 the upper limit of the waves. But it is especially upon the mud 

 flats of an aggrading stream or of temporary lakes, especially 

 in the drier regions of the country, that very many and perfect 

 ripple marks, mud cracks, and rain-drop imprints are formed 

 and stand best chance of preservation. Such are the very 

 regions where organic remains stand very slight chance of pres- 

 ervation, for the periodic removal of the water causes the rapid 

 oxidation of all lifeless organic matter, so that the principal 

 fossils present in such strata are tracks, trails, and burrows. 



Distortion of fossils. — In the bending, twisting, slipping and 

 crushing of portions of strata usually consequent upon upheaval 

 of great thicknesses of sediment from a region of deposition to 

 a region of erosion, more or less distortion of the included fossils 

 must occur. At times, merely the great 

 weight of the overlying sediment causes 

 distortion. So it is necessary to criti- 

 cally examine the fossil and, if possible, 

 compare it with others of the same 

 species to fully determine its original 

 shape. 



Pseudo-fossils. — The slipping and 

 twisting of strata likewise produce 

 forms that at times closely resemble 

 organic remains. These pseudo-fossils 

 are especially noticeable in metamor- 

 phic rocks. The slipping of one rock 

 face against another along a fault 

 plane produces slickensides which are 

 often closely similar to the Carbonif- 

 erous plant, Cordaites, or at times to 

 Calamites ; but the surface of a fossil will be parallel to the 

 lamina, whether it is the organism itself or its external mold 

 since it could normally have been deposited only in a position 



Fig. 6. — Dendrite (X 15), 

 a branching incrustation, 

 usually of manganese oxid. 

 It reveals its inorganic na- 

 ture in the complete ir- 

 regularity of its branching. 

 Drawn from the fractured 

 surface of a Pleistocene shell. 



