Vol. 6S.'] SUCCESSION IN THE NORTH-WEST OP ENGLAI^^D. 457 



•oolitic, while microscopic examination of numerous beds shows that 

 foraminifera contribute largely to the calcareous portion of the 

 formation throughout. A very interesting feature of the zone is 

 the important part played by calcareous algse as rock -builders, 

 many layers at certain horizons being largely made up of the 

 thalli of Solenopora and other more obscure forms. 



The source of origin of the magnesium carbonate which is so 

 constantly present in these beds is a question of some interest, 

 and we may briefly review here the evidence bearing on this point, 

 derived chiefly from the Shap District. This magnesium carbonate 

 must either have been deposited contemporaneously with the rest 

 of the rock, or have been introduced at some subsequent date. 

 The latter is apparently the view taken by the officers of the 

 Oeological Survey, for they remark : 



' In many places it [the limestone] is more or less dolomitic, apparently 

 through the introduction of magnesium carbonate at some period subsequent 

 to its deposition.' 



If this view be adopted, the most likely source of the magnesium 

 salts would be the waters of the Permian sea, but the Knipe-Scar 

 Limestone, which overlies the Orton Sandstone, is practically 

 devoid of magnesium carbonate ; and it is difficult to see how the 

 magnesium salts could have penetrated to the lower bed without 

 affecting the overlying mass of limestone. If, on the other hand, 

 the dolomitization was practically contemporaneous with the 

 deposition of the beds, the magnesium salts must have been present 

 in the Lower Carboniferous sea in which these limestones were 

 deposited. That some shrinkage has taken place in the rocks 

 since their deposition is shown by the abundance of drusy cavities 

 present in the beds, and such shrinkage is well known to result from 

 the conversion of calcareous rocks into dolomites. This shrinkage, 

 however, would occur in any case, whether the dolomitization took 

 place during Lower Carboniferous times or at any subsequent 

 period. An interesting piece of eyidence supporting the conclusion 

 that dolomitization took place under the waters of the Lower 

 Carboniferous sea, is furnished by the presence of drusy cavities in 

 the Coniston Limestone, which is exposed immediately below the 

 basement Carboniferous rocks in Blea Beck near Shap Wells. 

 This limestone, which must have been exposed to the waters of 

 the Lower Carboniferous sea, contains a considerable proportion 

 of dolomite, apparently of secondary origin, and it is difficult to 

 attribute this dolomitization to alteration during Permian times. 

 This rock has been described by Mr. Harker & Dr. Marr.^ It 

 encloses pure white crystalline patches up to a quarter of an inch 

 in diameter, which consist of clustered rhombohedra of unusually 

 pure dolomite,^ the carbonates of lime and magnesia being 

 present in the rock in about equal proportions. Farther west, 

 however, on the other side of the granite, where metamorphism 



1 Q. J. G. S. vol. xlix (1893) p. 367. 



^ See A. Harker, « Petrology for Students ' 4th ed. (1908) p. 271 & fig. 72. 



