GEOLOGY OF THE BRISTOL COALFIELD. 



41 



consider it a separate formation, but only a littoral deposit on a 

 Triassic seashore. 



The government surveyors have marked it on their maps. This 

 is not only very convenient, but there is considerable reason for 

 this determination because its composition is unique, and so far as 

 I know is not found in any part of the geological record. Dolomite 

 is a singular mixture of the Carbonate of Calcium and Mag- 

 nesium in their atomic proportion. The following is an analysis 

 of the dolomite of our neighbourhood — 



Clevedon. 

 Calcic Carbonate - 51.59 

 Magnesia, Carbonate 43.96 

 .22 

 .08 

 4.15 



Ferric Oxide - 

 Insoluble Matter 



Moisture, &c. 



CHfton. 



48.26 



41.12 



4.14 



2-34 

 4.14 



Good specimens may be collected from Clifton, Portishead, 

 Westbury or Clevedon. Its colour is commonly yellow or red 

 from the presence or absence of iron. 



The presence of the blocks and fragments of rocks is not 

 imperative. In Fig. 21, the upper part of the conglomerate is 

 massive dolomite inclosing and covering up the debris. At 

 Clevedon a remarkable instance of this may be seen at any time 

 where the Carboniferous limestone is covered up completely with 

 pure magnesian limestone, having a delicate buff colour and Oolitic 

 character. This bed is shewn in the accompanying woodcut, and 

 extends from Lady Bay to the hotel at the end of the beach. 



Fig. 21. Dolomite at Clevedon. 

 a b c * 



7^ 



a — Devonian, b — Lower Limestone Shales, c — Dolomite. 

 Wherever the Dolomitic conglomerate occurs, especially when 



