ORIGIN OF THE DOLOMITES. 195 



is also proved by chemical analyses of the rocks themselves, many ex- 

 amples'^ of which have elsewhere been brought together. 



In contrast with the results obtained from the foregoing specimens, 

 which seem to be normal de{)Osits for the material given, we note in some 

 exceptional cases quite different results. George Hughes reports'!* from 

 the island Aruba, West Indies, a mass of phosphatized coral whole 

 cargoes of which will test over 76 per cent of calcium phosphate. Dana X 

 also noted on Howland's island a pseudomorph consisting of phos- 

 phatized coral yielding 70 per cent of calcium phosphate. The same 

 author^ also found in the coral island of I\[ateo a sample which 3'ielded 

 CaCOs 61.93 per cent and MgCOg 38.07 per cent. This was probably 

 deposited in a lagoon where the several magnesium salts of the water 

 must fall down as precipitates, as the inclosed water was repeatedly 

 evaporated. 



Geographic Considerations. — The geographic conditions of any area must 

 be taken into account in searching out geologic details. The position 

 with reference to the sealevel is an especially important one. 



Continental Movements. — There is much evidence to show that the cen- 

 tral area of the North American continent has been uplifted above the 

 sea for a large portion of the time since the closing e})ochs of the Paleo- 

 zoic era. What can take place in accumulations lying fathoms below 

 the sealevel geologists and chemists are not yet completely al)le to say, 

 but where any marked changes are found to occur it is difiicult to 

 conceive of their not having involved the addition of material and a 

 thickening of the strata. Districts may be cited where within recent 

 geologic times probably no material change has taken place. The Aus- 

 tin limestone of Texas shows in three analyses reported by the state 

 survey II only a trace of magnesium salts. In another i)lace it is re- 

 ported that the Austin-Dallas chalk varies from 85 i)er cent to 94 i)er 

 cent calcium carbonate and 21 per cent, more or less, of magnesium car- 

 bonate,*)^ as calculated from the analysis of the rock at hand. The above 

 are cited as instances — perhaps fair ones, yet in some resi)ects parallel 

 ones — of a Mesozoic limestone which was accumulated and probably kept 

 below the surface of the sea much of the time until the Pleistocene epoch. 

 It is a.s a formation of great extent and of universally horizontal position 

 that it has l^een raised to its present altitude. W^ith the beginning of the 

 existing conditions a new series of chemical phenomena is oi)ened whose 



♦ Bull. Geol. Soo. Am., vol. 3, 18'.>2, p. 348. 



tQiiarterly Journiil of the Oeolonical Society, 188.'», vol. xli, p si. 



tCofftls unci Coral Islamln, 1871, p. 293. 



g Ibid., pp. .34'!, :«7. 



IGeolovcionl Survey of Toxft.s, Tiiinl Ami. Ii<'p., IHiil, jtp. :',:>l -:',:> I. 



^ Fli>t Ann. Il«'p. Geol. Survey of Texiv-s, 1.VJ0, j). ll.J. 



