GEOLOGY. 329 



to the rock, but usually in more or less rounded, irregular pieces, imbedded 

 in a red ferruginous day. Both the dolomite and the red clay contain small 

 quantities of carbonate of zinc; the dolomite, in some localities, nearly 2 

 per cent., the red clay only 0'38 per cent. At another locality, occurs, asso- 

 ciated with galena, a very light and soft day, of ochre-yellow color, which 

 contains rather more than 8 per cent, of oxide of zinc. 



The Smithsonite of Marion county is occasionally found incrusted with a 

 white hydro-carbonate of zinc, whose composition is expressed by the formula 

 3(ZnO.C02)+2(ZnO.3HO) and for which the name of Marionite is proposed. 

 It contains nearly 4 per cent, more oxide of zinc than Smithson found in 

 his analysis of zinc bloom. 



The same chemist publishes analyses of three specimens of psilomelane, 

 from Independence county considering them, with Rammelsberg, as com- 

 pounds of peroxide of manganese with bases of the constitution RO. He 

 derives from his analyses the following formula, as expressive of their com- 

 position. 



(MnO.BaO.HO).MnO2-j-6"37 per cent, of MnO-2 mechanically intermixed, 

 and (MnO.BaO.CaO.HO).MnO2+3'84 per cent, of MnOa mechanically inter- 

 mixed. 



A specimen of wad, from Izard county, has the same composition as the 

 wad from Rubeland, analyzed by Rammelsberg, namely. 

 (MnO.CoO.BaO).2MnO2-r-3HO+xMnO2 



The composition of a fifth specimen of manganese ore is expressed by the 

 formula 4(MnO.CaO.MgO.HO).3Mn02; or, if the small quantities of lime, 

 magnesia, and water are considered as accidental impurities, by MmOs, the 

 composition of bramite; the proportion of manganese to oxygen being as 

 69*08 to 29'72; in brannite the proportion is 69'G8: 30*42. 



Prof. Elderhorst expresses the opinion that Arkansas is destined to take 

 the lead of all the Western States ia her resources of ores of zinc and 

 manganese. First Report of a Geological Reconnaissance of the Northern 

 Counties of Arkansas. By D. D. Owen, Win. Elderhorst, and Edward T. 

 Cox. Little Hock, 18C8. 



PItOF. OWEN ON FOSSIL MAMMALS. 



The following is an abstract of the concluding lecture of a course recently 

 delivered by Prof. Owen, before the Royal Institution (London), " On Fossil 

 Mammals," in which the causes of their extinction are particularly con- 

 sidered. 



On the problem of the extinction of species, I have little to say; and of the 

 more mysterious subject of their coming into being, nothing profitable or 

 to the purpose, at present. As a cause of extinction in times anterior to 

 man, it is most reasonable to assign the chief weight to those gradual 

 changes in the conditions affecting a due supply of sustenance to animals 

 in a state of nature which must have accompanied the slow alternations of 

 land and sea brought about in the aeons of geological time. Yet this reason- 

 ing is applicable only to land animals; for it is scarcely conceivable that 

 such operations can have affected sea-fishes. 



There are characters in land animals rendering them more obnoxious to 

 extirpating influences, which may explain why so many of the larger spe- 

 cies of particular groups have become extinct, whilst smaller species of 

 equal antiquity have survived. In proportion to its bulk is the difficulty of 

 the contest which the animal has to maintain against the surrounding agen- 



28* 



