CAMBRIAN AND LOWER SILURIAN ROCKS. 553 



1000 feet. In Russia they are still of less thickness ; and there the 

 earlier deposits seem to be entirely absent. In Bohemia they seem 

 to occupy an almost intermediate position, as to thickness and order of 

 deposition, between those of Britain and those of the extreme north ; 

 and this is what might naturally be expected if my suppositions are 

 correct. However, to prove these facts it is necessary to examine 

 the series carefully in each area both by palseontological and strati- 

 graphical evidence. On the chart (Plate XXVII.) the order and 

 thickness of the deposits in each area are shown, and the character 

 of the sediment indicated. 



Taking the British Isles first, and making the "Welsh sections to 

 represent the general order over the areas to the south-west, we 

 find (1) at the base of the series beds of conglomerates consisting of 

 quartzy pebbles and other masses which are undoubtedly identical 

 in character with the rocks composing the underlying ridges of prae- 

 Cambrian rocks, and indicating beach-conditions. Following these 

 (2) are sandstone beds ripple-marked, or shallow-water accumula- 

 tions ; (3) finer beds are thrown down, indicating that a somewhat 

 greater depression had rather suddenly taken place, and the earliest 

 organisms known in this region are here entombed, having been evi- 

 dently driven in from the deeper water to the south-west, where 

 probably they had attained through successive changes the stage of 

 development then shown ; (4) the depression was very gradual for a 

 long period, and the beds were deposited in shallow water, being for 

 the most part sandstones ; (5) a more sudden depression takes place 

 and finer beds are thrown down, in which our second fauna occurs ; 

 (6) another quiet and gradually subsiding period, which, however, is 

 followed by (7) a very decided depression, in which our third fauna 

 occurs. This depression, which may be called the " Menevian," and 

 which, taking the whole epoch, caused alone a change of level of 

 probably over 1000 feet, when added to the 8000 or 10,000 feet 

 which had already taken place, allowed the waters to spread freely 

 over the areas extending between the south of Prussia and Bohemia, 

 and Norway and Sweden. There is no evidence of the presence of our 

 first and second faunas in any of the regions so far to the north and 

 east, the lowest beds in Norway and Sweden being probably equivalent 

 only to our Menevian group ; and the evidence is strong also that the 

 first Bohemian fauna was the passage onwards of our third or early 

 Menevian fauna, and that the waters did not reach the so-called 

 Bohemian basin until most, if not the whole, of our Longmynd 

 group had been deposited. This depression was not sufficient, how- 

 ever, to allow the waters to spread over the Russian areas ; therefore 

 we have no rocks of this age there. Coming back again to our 

 British areas, we find that our last depression has been very gra- 

 dually filled up, and shallow-water accumulations are again deposited. 

 During the next, or Lingula-flag period, shallow- water accumulations 

 were almost universally thrown down ; and though they include 

 over four thousand feet of beds, and consequently indicate a de- 

 pression to that extent, yet in no case do we find, until we come 

 towards the close of the epoch, the depression occurring at a greater 



