368 Mr. D. Forbes on the Chemical Composition of 



Carbonate of lime 79'97 



Carbonate of magnesia .... 0*52 



Sesquioxide of iron and alumina . 0'83 



Phosphoric acid . . . . . . 0*56 



Organic matter 0'56 



Insoluble 17-85 



100-28 



5. Cambrian. — Limestone from Church Strettou, Longmynd. 



In a cursory examiuation of this district last winter, I found 

 a thin bed of limestone probably from 4 to 6 inches in thickness, 

 rather irregular, and occurring imbedded in the brownish-gray 

 clay-slates immediately in the village of Church Stretton, Owing 

 to the weather and untoward circumstances it was but very su- 

 pei-ficially examined. 



This limestone is not marked upon the Ordnance Geological 

 Survey Sections of this district (Nos. 34 and 36), nor on the 

 Map (61 S.W.), and, as far as I am aware, has not previously 

 been noticed. It is of a bluish-gray colour, compact, and shghtly 

 crystalline ; in parts it is often very pure, frequently much more 

 so than the specimen analysed. 



This limestone must probably be at present considered as the 

 lowest known limestone whose geological position can be cor- 

 rectly determined; and by comparison with the Ordnance Geo- 

 logical Survey Section (No. 36), it must be situated more than 

 23,000 feet below the base of the Llandeilo or Bala beds. No 

 traces of fossils were observed ; but when dissolved in acids, the 

 piece analysed left a siliceous cast, very similar to what might be 

 expected when dissolving a coraUiue limestone where the inter- 

 stices of the coral had subsequently been filled up by siliceous 

 matter. 



The specific gravity was found to be 2*33 at 60° F. The quan- 

 tity operated upon was 23-61 grs., which afforded 6-37 insoluble 

 skeleton, 0-08 organic matter, 2-14 sesquioxide of iron and alu- 

 mina with phosphoric acid, 0-20 phosphate of magnesia from 

 estimating the phosphoric acid, and 0-39 ditto from the magnesia 

 present ; also 14-90 carbonate of lime. 



The analysis when tabulated will stand as follows : — 



Carbonate of lime 63*10 



Carbonate of magnesia .... 0*80 



Sesquioxide of iron and alumina, 8-51 



Phosphoric acid 0-55 



Organic matter 0-33 



Insoluble 26-98 



100-27 



