Vol. 70.] OF DURHAM MAGNESIAN LIMESTONES. 249 



LXXVI. The typical dark-grey, soft, bituminous rock with fish- 

 remains. 



LXXV. LXXVI. 



Insoluble residue 12*48 41-18 



Bituminous matter 0*44 13*92 



FeCO, 0*67 0*56 



A1,0 3 5*15 1-87 



CaCO, 48-60 22-91 



MgC0 3 32-26 17-86 



CaSO.J t 0-46 1-83 



Totals 100-06 100*13 



Calcite 10-19 1-65 



Dolomite 70-67 39-12 



III. Evidence bearing upon Dolomitic Deposition and 



ON THE DOLOMITIZATION OF THE SHELL-LlMESTONE. 



I do not intend to raise any theoretical questions regarding the 

 origin of dolomites, in a paper intended to be purely a record of 

 observations. I may say, however, that during the course of the 

 present work the following considerations have suggested themselves 

 as evidence supporting the view of the direct deposition of the 

 greater part of the dolomite in the bedded Magnesian Limestones. 



(i) The general uniformity in dolomitic composition over wide 

 areas of most parts of the formation, as indicated by analyses of 

 those portions that have escaped segregation. In the dolomitic 

 Lower Beds fossils are either absent, or only very sparingly present. 

 In- some of the unaltered Middle Beds on the in-shore side of the 

 reef small forms occur in plenty. It is only in a few of the Upper 

 Beds that Schizochis, Liebea, and Chondrites occur in quantities, 

 approaching a rock-building character. In very few of these cases 

 do the organisms themselves seem to have become dolomitized, the 

 ■casts of the fossils being represented by clean hollow cavities, in 

 contrast with the dolomitic powder which fills the shell-spaces in 

 the Shell-Limestone. When rapidly covered up with sedimentary 

 material, the organisms seem to have escaped dolomitization. The 

 perfect preservation in pure dolomites of casts of such minute forms 

 as ostraeods shows that the probability of a wholesale obliteration 

 of organisms through dolomitization having taken place in these 

 bedded dolomites, is a very remote one. No trace of ' patchy 

 dolomitization,' except in parts of the reef, has been observed. 



(ii) Those beds that enclose hollow spaces, which I endeavoured 

 to show 1 were left by cr3 r stalline anhydrite-aggregates, consist in 

 every case of nearly pure dolomite. If dolomitization had been 

 effected through a secondary process, it would seem probable 



1 ' On a Mass of Anhydrite in the Magnesian Limestone at Hartlepool ' 

 •Q. J. G. S. vol. lxix (1913) pp. 196-96. 



s2 



