756 APPENDIX. 



species that have no external shell. The earliest of this kind were probahly the 

 Gonularise, as already stated. 



Page 597. — In the sub-kingdom of Radiates there are two comprehensive 

 types intermediate between the classes of Polyps (the inferior) and Acalephs 

 (the next higher). (1.) That of the Cyathophylloid corah, whose fundamental 

 structure is based on the number 4, the number eminently characteristic of Aca- 

 lephs, the rays being multiples of four, and not (as in modern corals) of six. 

 (2.) That of the Favosites family of corals, which, if related to modern Mille- 

 pores and true Acalephs, as Agassiz holds (p. 162), belong to that division of 

 Acalephs which embraces species so polyp-like that until recently they were 

 arranged with Polyps. The species of these comprehensive types are the only 

 known representatives of Polyps and Acalephs in the Palaeozoic faunas, except- 

 ing possibly the Graptolites; and these, if Acalephs (p. 190), are Hydroids like 

 the Favosites. 



Coral-making Mollusks (Bryozoans), Polyps, and Acalephs, were a prominent 

 part of that harmonious assemblage of groups which constituted the life of the 

 Palaeozoic. 



L.— Mineral Oil. 



Petroleum has proceeded from the decomposition of vegetable or animal 

 remains, and mainly the former. These remains (marine, or terrestrial) were 

 buried in the mud forming the bottom of shallow waters, and there, the air 

 thus excluded, were slowly converted into oil, or into some similar material 

 yielding it on distillation. Petroleum consists of carbon and hydrogen: and 

 in the change to it, woody fibre loses its oxygen. When the oil is afterward 

 exposed to the air it often oxydizes, and becomes thick, and finally solid, passing 

 ultimately to asphaltum or some analogous compound. It is sometimes (as well 

 illustrated in California) thin flowing oil from the lower part of a stratum, and 

 thicker from the upper where the air may have had access ; and solid, or nearly 

 so, where it has come out into the open air. 



The rocks affording it are shales, sandstones, and limestones, or such rocks as 

 are made by the consolidation of the mud (either argillaceous, arenaceous, or 

 calcareous) of sea-bottoms. Shales and argillaceous sandstones are the most 

 common original source. The oil found in arenaceous sandstones is supposed 

 to have been derived from the shales above, or below. 



In some regions (as observed in Pennsylvania, Michigan, California) the oil 

 exudes freely from the shaly or other oil-bearing rock into openings made by 

 boring or tunnelling, whence it may be gathered. In other cases, the oil exists 

 already in subterranean chambers confined above by impervious strata ; it having 

 been accumulated in such open spaces, either by simple exudation from oil- 

 bearing rocks adjoining (or not far above or below), or by distillation or vapor- 

 ization from a rich subjacent stratum. The oil in such chambers is usually 

 under pressure from imprisoned gases derived from the same source as the oil; 

 and in this case, when a boring is made into it, it rises in the well toward the 

 surface, and sometimes is thrown out in a jet of great height. These chambers 

 are most likely to exist, along gentle anticlinals ; but they may be a result of 

 erosion, like ordinary caverns, or of some other agency. 



Oil-bearing rocks are of all ages. Lower Silurian limestones afford flowing 

 wells in Kentucky, Tennessee, and the Manitoulin islands. Upper Silurian wells 

 exist on Boyd's Creek, Kentucky. The Devonian supplies wells at Enniskillen 

 in Canada West, which arise either from the Corniferous limestone (Hunt), or 



