16 SKETCH OP THE GEOLOGICAL HISTORY OF THE BRISTOL DISTRICT. 
Europe. Slowly the waters of this southern sea stole northwards, 
until at last they invaded the English Kenper lake. And with them 
came marine reptiles, Ichthyosaurus and Plesiosaurus^ and hshes such 
as Hyhodus^ Saurichthys, and Ceiritodus. The remains of these 
reptiles and fish occur in the greatest abundance in the Rhsetic Bone 
Beds, so well known at Aust and elsewhere in the Bristol district. 
These interesting deposits, though showing much variability, are, as 
a rule, breccias or conglomerates consisting of fragments of the 
neighbouring Palaeozoic rocks mingled with bones, teeth, scales, and 
coprolites, the whole being embedded in a matrix which is generally 
calcareous, or pyritous, sometimes sandy. The character of both 
fragments and matrix varies with that of the underlying Palaeozoic 
rock. 
As regards the origin of these curious deposits, the explanation 
most generally given is that the vertebrate immigrants from the open 
Rhsetic sea to the south, entered the area of the old Keuper lake 
before the saltness of the water was sufficiently reduced to form a 
suitable medium in which they could live, with the result that they 
died in thousands and their remains formed the bone beds. 
This theory affords no explanation of the facts that more than one 
bone bed may occur at different levels in the series, and that the 
bone beds are not invariably at the base of the series. Mr. W. H. 
Wickes ^ has brought forward an ingenious explanation regarding the 
bone beds as due to shifting shoals of fish and of the reptiles which 
preyed on them. His theory not only offers an explanation of the 
variability in number and position of the bone beds, but also of their 
irregular distribution and of the fragmentary character of the vertebrate 
remains, nothing approaching a complete skeleton being ever found. 
We have divided the geological history of the district into chapters, 
in each of which is recorded a definite and continuous episode in 
evolutionary development. The first chapter is occupied with the 
formation of the Palieozoic strata, the Silurian, Old Red Sandstone, 
and Carboniferous. The second is concerned with the uplift, under 
severe earth pressures, of these rocks, with their ridging into arches 
and troughs and with their early fashioning under the tools of 
denudation. The third deals with the burial of the carved surface 
under the newer Mesozoic sediments. First, the denuded lowlands 
were occupied by the waters of the Keuper lake ; these then crept 
up the sides of the hills, and formed creeks in their valleys, until 
only an archipelago of islands broke the surface of the lake. Then 
the sea gained access, and the Rhsetic beds were formed in the waters 
around the sinking archipelago. As Lias times followed, the islands 
disappeared, and were covered by deposits ^ of this age, the lithologi- 
cal character of which — often a white or light-grey limestone of 
slightly crystalline texture— may well puzzle the geologist accustomed 
1 ‘Proc. Bristol Nat. Soc.’, New Series X., pt, 3 (1904 issued for 1903), p. 213. 
2 See C. Moore, ^Q. Journ. Geol. Soc.’, XXIIL (1867), p. 449. 
