458 PEOF. r. J. GARWOOD ox THE LOAVEE CAEBOXIFEEOTJS [DeC. I912, 



of the Conistoii Limestone has taken place, the proportion 

 of magnesia to lime is only : : '3'Qo : 11-50; this may be taken as 

 the original relative proportion of these salts present in the 

 limestone. 



The bulk of the rock in Blea Beck, as shown by ^Ir. Harker, 

 co])rains magnesium carbonate in addition to that found in the 

 crystalline patches. The former may represent the magnesium 

 originally present, corresponding to that found in the Coniston 

 Limestone altered by the granite farther west ; while the latter 

 would then represent additional magnesia introduced after the 

 intrusion of the granite, and therefore in Lower Carboniferous 

 times. As shown by recent researches, when silicates are formed 

 by metamorphism of magnesian limestones, the magnesia is always 

 converted into silicates much more readily than the lime, and the 

 abundance of lime-silicates in the metamorphosed portion of the 

 Coniston Limestone seems to show that the rock contained but a 

 small proportion of the magnesia before the intrusion of the 

 granite. 



It seems unlikely that the only outcrop of the Coniston Lime- 

 stone known to contain a high percentage of crystalline dolomite 

 should immediately underlie the Lower Carboniferous rocks (which 

 also contain similar dolomitic geodes), unless the material of the 

 geodes in both cases was derived from a common source. This 

 fact, taken together with its absence from the overlying lime- 

 stones, seems to point conclusively to the doloraitization having 

 occurred in early Carboniferous times. It appears probable that 

 this dolomitization is an expression of the shallow-water conditions 

 which then prevailed. The deposition of the Shap dolomites in 

 the depressions of the pre- Carboniferous land-surface suggests that 

 lagoon conditions may have obtained during their formation, and 

 in this connexion the reef-like masses of calcareous algse, so 

 characteristic of the lower portion of these beds, are of interest. 

 Eecent work on the origin of dolomite seems to point to the 

 conclusion that the conditions most suitable for dolomitization 

 occur in association with coral-reefs, in the formation of which 

 calcareous algoe have been shown to play a conspicuous part. 



In this connexion it is interesting to note that the other 

 exposures of this horizon in the ^sorth-^^estern Province, namely 

 those in the Ravenstonedale, Arnside, and Kendal Districts, are also 

 rich in magnesium carbonate, and are again characterized by the 

 abundance of Soleno/iora and other calcareous algoe. 



We may conclude, then, that the dolomitization of the beds 

 of the Atliiiris-(ilahristria Zone was practically contemporaneous 

 with the deposits, in the sense that the coral-rock of the Funafuti 

 Atoll and its dolomitization may be regarded as contemporaneous ; 

 that it took place under lagoon conditions in a subsiding area 

 formed of the irregular pre-Carboniferous land-surface ; and that 

 these conditions were favourable to the growth of calcareous algae, 

 as in the case of recent coral-reefs. 



