44 



GEOLOGY OF THE BRISTOL COALFIELD. 



Immediately on the conglomerate, lie sandstones and variegated 

 marls of the Keuper period. In Fig. 24 is the sketch of the good 

 section that is displayed at the sides of the new river near the gaol. 



Near Compton Dando, the shafts of the coal pits have to be 

 sunk through 210 feet of these marls and sandstones, before reaching 

 the " overlie.'' The harder beds of the sandstone are unfossiliferous, 

 and are used for building purposes. The cathedral and many of 

 the old buildings were constructed of these rocks. 



On these beds rest the marls coloured blue or red according to 

 the oxide of iron they contained. The red is caused by the 

 peroxide, while the blue and green colours are caused by 

 the presence of the iron as protoxide or carbonate. 



On the top of the sandstone beds, and embedded in the marls is 

 the great deposit of sulphate of strontian or celestine. It occurs 

 as small crystals disseminated through the marls or in nodules or 

 blocks varying in weight from a few ounces to more than a ton. 

 The crystals are tabular, and have either a pure white, pink or 

 delicate blue colour. The last is said to be car.sed by the 

 presence of phosphoric acid, but I have not been able to confirm 

 this by experiment. Fig. 2 j gives a sketch of a section in Oakfield 

 road, where the position of the celestine is well shewn. 



Fig 25. — Section in Oahfidd Road. 



a — Keuper marls with boulders of celestine. b — Keuper Sandstone. 



Frequently as at Pyle Hill, Gotham, Warmley, Bitton, and at 

 the N.W. end of Durdham down tunnel, magnificent geodes have 



