164 NOTES ON TRIAS DYKES. 



lis, nor, although they seem to run in two directions, have we 

 been able to notice that one set lifts the other, or was formed at a 

 subsequent date. 



In cutting the Tunnel under the Downs through the 

 Carboniferous Limestone, fine crystals of Celestine were obtained. 

 Now the jS'ew Eed of Bristol has long been famous for the 

 Celestine found in it, and from the description of our collector 

 (J. Owens) I have little doubt but that the deposit met with in 

 the Tunnel was from a fissure filled in at the time of the ]!^ew Red. 

 The blocks to which the crystals were attached were large blocks 

 of Carboniferous Limestone, some half a ton in weight, and were 

 rounded and corroded, but had the paste of tlie Magnesian 

 Conglomerate sticking to them occasionally ; hence it was evident 

 that the crystals were in cavities of Triassic Conglomerate which 

 filled a fissure, some six feet wide, in the Carboniferous Limestone. 



It would seem also, that we have evidence in these fissures of 

 deposits of later age th'an that of the ^Xew Red. There is one near 

 the Suspension Bridge which may be of Rhaetic or L. Lias age. It is 

 some time since the surface was cut, and it is not now very clear ; 

 but we see what may be a wide fissure (or pocket) in which 

 blocks of Cotham-marble are found imbedded. Lias Septaria too 

 were dug out of it in making the road to the bridge and the 

 excavations for the bridge chains. The bulk of the infilling 

 material seems to be greenish marl, such as occurs in the Rhaetics, 

 with a little red marl: the fissure was probably filled up in the 

 time of the Lias. Professor Jukes'^' has remarked, that the Lias 

 must have extended over all this district as it occurs on high 

 ground round Westbury. 



Supposing then that the Trias and Lias covered the 

 Carboniferous Limestone, the question may arise, was the river 

 gorge in existence at that time ? An examination of the mode of 

 occurrence of the Dykes wall incline us to give a negative answer. 

 We may then with more confidence attach ourselves to the 



* Geol. Mag. IV. p. 445, 1867. 



