LAUKENTIAN AND EARLY PALEOZOIC. 21 



vegetable kingdom so many of them as some of his coa- 

 temporaries.* The considerations which seem most im- 

 portant in making such distinctions are the following : 

 1. The presence or absence of carbonaceous matter. 

 True AlgBB not infrequently present at least a thin film of 

 carbon representing their organic matter, and this is the 

 more likely to occur in their case, as organic matters 

 buried in marine deposits and not exposed to atmospheric 

 oxidation are very likely to be preserved. 2. In the 

 absence of organic matter, the staining of the containing 

 rock, the disappearance or deoxidation of its ferruginous 

 colouring matter, or the presence of iron pyrite may indi- 

 cate the removal of organic matter by decay. 3. When 

 organic matter and indications of it are altogether absent, . 

 and form alone remains, we have to distinguish from Algse, 

 trails and burrows similar to those of aquatic animals, 

 casts of shrinkage-cracks, water-marks, and rill-marks 

 widely diffused over the surfaces of beds. 4. Markings 

 depressed on the upper surfaces of beds, and filled with 

 the material of the succeeding layer, are usually mere im- 

 pressions. The cases of possible exceptions to this are 

 very rare. On the contrary, there are not infrequently 

 . forms in relief on the surfaces of rocks which are not 

 Algse, but may be shallow burrows arched upward on top, 

 or castings of worms thrown up upon the surface. Some- 

 times, however, they may have been left by denudation 

 of the surrounding material, just as footprints on dry 

 snow remain in relief after the surrounding loose material 

 has been drifted away by the wind ; the portion consoli- 

 dated by pressure being better able to resist the denuding 

 agency. 



The footprints from the Potsdam sandstone in Can- 

 ada, for which the name Protichnites was proposed by 



* " Impresaions and Footprints of Aquatic Animals," " American 

 Journal of Science,'' 1873. 



