Page:Popular Science Monthly Volume 14.djvu/707

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uppermost strata furnished by their decomposition the carbonaceous material in the associated rocks, the heat derived from the slowly-cooling injected rocks playing an important part in this process. Similar deposits of carbonaceous mineral in igneous rocks are found in other localities, as at Cape Gaspé, and in the lava of Mount Etna, though in these cases it occurs in the less concentrated form of mineral oil. But Mr. Russell sees in these different forces only different stages of one and the same process. "If," he writes, "the cavities of a rock were filled with petroleum by infiltration, and evaporation slowly removed the more volatile portions, and oxidation took place to some extent, the result would be the formation of a deposit of solid hydrocarbon in the cavities. A similar process sometimes occurs with bottled samples of petroleum, by which the interior of the bottle is left coated with a solid carbonaceous layer. In the rocks, if a fresh supply of oil were furnished from time to time by infiltration, the cavities would eventually become completely filled with the solid carbonaceous residue. A vesicular lava might in this manner be changed to an amygdaloid, the cavities of which would be filled with solid hydrocarbons instead of quartz, zeolites, etc. Such, it appears to us, must have been the history of the Triassic amygdaloid we have described, the cavities of which must at one time have been filled with mineral oil. This is but an epitome of what took place on a grand scale at the great fissure, over 1,400 feet deep, in New Brunswick, which was filled with albertite, and in the case of the Grahamite in West Virginia, which also occupies an immense fissure."


Whence came the Arctic Mammoth?—Were the mammoths whose remains are found in the north of Siberia native in that region, or were they carried thither by rivers from a more genial climate? If the latter supposition were correct, we should find the remains only in the vicinity of the great watercourses, while, in fact, they occur in localities distant from the beds of streams. But here fresh difficulties face us, for, though the mammoth was covered with a thick coat of long hair, how could it live in a region where the temperature in Januuary is as low as 65° Fahr., where the summer lasts only three or four months, and where the vegetation is exceedingly scanty? The supposition is plainly inadmissible, and hence either we must believe these remains to have been transported hither, or else that in early times the climate of Siberia was much less severe than it is at present. Nor is this a baseless theory, for, as Mr. U. H. Howorth observes, the plants found in the fissures of the rhinoceros-teeth (contemporary with the mammoth) are those which now live in southern Siberia. The plant-remains associated with the mammoth (not floated from a distance, but of the locality) show the same thing, the species being larch, birch, and other trees of good size. Other evidences of the existence of a higher mean temperature in Siberia at the time of the mammoth are found in the fresh-water and land shells associated with the remains, but now extinct in northern Siberia. As for the manner of the mammoth's extinction, Mr. Howorth believes it to have been sudden. The remains must have been preserved soon after death. They were destroyed by a flood due to some sudden convulsion, which also changed the climate.


Civilization and Teeth.—From the study of 1,249 skulls, of which 844 represent modern highly civilized races, and 277 modern inferior races, while the remaining 128 belonged to Romans, Etruscans, Phœnicians, and other nations of antiquity, Professor Mantegazza reaches conclusions which go to confirm a remark made by Mr. Darwin in "The Descent of Man." "It appears," writes Darwin, "as if the posterior molar or wisdom teeth were tending to become rudimentary in the more civilized races of man." Professor Mantegazza finds that the wisdom-teeth are more frequently absent in the superior than in the inferior types, the exact proportion being 42·42 per cent, in skulls of the higher races against 19·86 in the lower. But atrophy of the third molar tooth occurs less frequently in the higher than in the lower races, viz., in 10·90 per cent, of the former, and in 20·58 per cent, of the latter. In the lower races the abnormal cases are practically equal to the normal, while in the higher they are much