-JMICHOLS. 49 



O.486 

 O.189 



3-59 

 o.45 2 



7.644 



4-455 

 1-349 



11.72 



o 118 

 0.401 



5-5 



It may also be noted that Sharpies when in 187 1 he determined 

 the phosphate in seven zoanthoid corals observed that traces of 

 magnesia were present in all, but made no quantitative determina- 

 tions. A. Damour*! also found small quantities of magnesia in many 

 millepores. 



From the above results it appears that the algae, crinoidea, 

 vermes and alcyonaria secrete relatively magnesian skeletons, while 

 the zoantharia, pelecypoda, gastropoda and cephalopoda secrete 

 skeletons which are only slightly magnesian. 



These analyses thus explain why the inner portion of the nodule 

 analysed by the author, (p. 45) is less magnesian than the outer part. 

 This nodule like many of the others was formed in and around a 

 large gastropod. The more highly magnesian corals, serpulae and 

 algae of which the nodule is composed are in the central part diluted 

 by the less magnesian gastropod material. It is probable that the 

 magnesium of the outer part is also somewhat increased by that 

 re-solution of the skeletal material which is always taking place. 



If under present conditions corals, etc., secrete skeletons which 

 may contain over ten per cent carbonate of magnesia, may they not, 

 under palaeozoic conditions, when, as is usually conceded, the sea 



tAnalysis by T. Sterry Hunt : Logan's Geology of Canada, 1863. The ash analyzed was 61 per 

 cent of the whole shell and gave 2.88 per cent Mg O, whence the equivalent Mg CO3 for the entire 

 shell has been calculated. 



tCirri and pinnulate arms from an alcoholic specimen. The organic matter is 22 per cent. 



^Analysis by Gumbel; Geikie: Textbook of Geology, p. 482. 

 Sharpies: Am. J. Sci., Ill ser., vol. I, p. 169. 



c Dana: Manual of Geology, p. 72. 



