18 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



materials tell little of the place from which they came. Dunes may form 

 on the edge of lakes or oceans, or in the most stark desert, and in either 

 place the information given by the structure and material of the dune would 

 be essentially the same. The constituent sand might be gathered from 

 barren mountain peaks, dragged from a normally well-covered region in 

 time of drought or failing vegetation, or swept from river bars, lake or ocean 

 beaches. 



Loess and volcanic ash tell even less than aeolian sand. These are so 

 light that they may be transported far from their place of origin and laid 

 down in the most dissimilar places, in bodies of water, on lands covered 

 with vegetation, or in deserts. 



c. The mineral content is instructive only where conditions are most 

 favorable. No one would be able to interpret from the deposits of a swamp 

 in the Archezoic rocks of Canada the nature of its surroundings if the 

 chemical content of the finer muds were alone observable. A thoroughly 

 decomposed mass derived from various igneous rocks would give upon 

 analysis results very similar to those obtained from many shales, but if the 

 mineral content is still determinable by petrographic methods, or if coarser 

 deposits on the borders of the swamp or near the inflowing streams are 

 observable, much might be learned. 



An occurrence of scattered local deposits of fine-grained shale rich in 

 carbonaceous material or plant remains, accompanied by fresh-water fossils, 

 and perhaps by a quantity of bog iron ore in association with pebbles of 

 igneous rock in advanced stage of chemical decomposition would strongly 

 suggest such conditions as now prevail in many parts of Canada. An 

 abundance of angular fragments of similar igneous rocks with swamp 

 deposits would suggest the former existence of such swamps as occur in 

 the higher mountain parks, while, of course, striated pebbles would lead to 

 the consideration of the possibility that glaciers had had some part in the 

 formation of the swamp by interfering with established drainage. 



A coal swamp in a limestone region would be even less liable to disclose 

 the nature of the surrounding land, especially if it were of large size. The 

 fragments of limestone carried into the swamp would disappear by solution 

 and the infrequent stream-channels would retain such a relatively large 

 residium of the insoluble material, as quartz sand, as to lead to erroneous 

 conclusions, unless studied with the utmost circumspection. River deposits 

 would be far less dependable as indices to the nature of the surrounding 

 lands than deposits in bodies of quiet water. Large streams carry material 

 for great distances, and the content of any fossil river-channel might be the 

 result of accumulations from widely separated sources. The presence of 

 igneous fragments in the middle or lower courses of the Mississippi would 

 obviously not be a safe proof that the shores adjacent to the place where the 

 samples were taken were formed of igneous rocks. Floating ice might 



