GEOLOGY OF THE BRISTOL COAL-FIELD. 337 



The Millstone Grit follows so gradually and conformably that it 

 and the shales are sometimes nndistingiiishable. Generally speaking, 

 the Millstone Grit beds are very thick and solid, as may be seen on 

 the slopes of Brandon and Clifton hills. The Grit is formed of the 

 hardest crystalline grains of sand, agglomerated with oxide of iron. 

 In some places the iron is so abundant that the colour is a dark 

 red, while in others it is a delicate pinkish grey. Some specimens are 

 homogeneous, others are prettily striated. So hard are some beds, 

 that they are preferred to Welsh Greenstone for paving stones where 

 the traffic is heavy. Frequently the Devonian Mica is plainly 

 visible. Sometimes a large percentage of limestone is met 

 with, and most probably owes its presence to the shells of 

 bivalves. In one of the lower Brandon-hill beds a large number of 

 ProductcB are seen, where the lime has totally disappeared, 

 having been dissolved oat, leaving a hollow cast of the shells. The 

 Millstone Grit is not generally very fossiliferous, only a few beds in 

 this neighbourhood being productive. Majoi Austin was fortunate 

 enough to find a large number of mollusca and fish remains in one 

 of the Tyndall Park beds, whence he obtained upwards of forty 

 species. The only other localities that I have found to be fossiliferous 

 have been on the north and south sides of the Avon, near llownham 

 ferry, the base of Brandon-hill, and near the fault in Leigh "Woods. 



The thickness of the Millstone Grit in this neighbourhood has 



been variously estimated. Mr. Etheridge puts it as 1200 feet, Mr. 



Anstie at 1000 feet, Professor Hull at 950 feet, while Major Austin 



estimates it at more than 2000 feet. Prom my own observations, I 



think 1000 feet is not far from the truth. However this may be, 



the Millstone Grit must have taken a very long period for its 



deposition. The regularity of the bedding, and the variation in the 



petrology and the occasional bands of limestone prove this. In 



the Millstone Grit are one or two seams of Coal, but quite 



unworkable. 



The Lower Coal Meastjees. 



The Coal-bearing strata of this neighbourhood are divided into 

 two sets, by an intervening band of grits called Pennant. The 



