59O SCIENTIFIC RECREATIONS. 



highly characteristic of those organisms, but when sections of them are made 

 sufficiently thin to be transparent, a spongy texture may be most distinctly 

 recognised in their interior." 



It is now generally admitted that the decaying animal matter acts upon 

 the silicious spiculae of sponges, etc., and the silica is thus deposited. 



We may then surmise that at some very distant period the whole 

 extent of the British Isles was submerged, as well as portions of the continent, 

 and after the strata had been deposited the sea and land were disturbed by 

 volcanic action. While the secondary strata were being deposited, very little 

 relative alteration took place, as the deposits are seen to lie " conformably." 

 But when the great convulsion which upheaved the Apennines occurred, the 

 chalk was raised as we find it in the cliffs and downs, which were the beds 

 of seas. This is the last of the great convulsions which the earth has 

 undergone, for the tertiary strata, which afterwards began to be deposited, 

 rest in the hollows or basins (chiefly in the chalk) then left ; the alterations 



Fig. 679. Mosasaurus (Maestricht). 



in and since these deposits appear to consist chiefly of the upheaval of certain 

 localities, the depression of others, the evaporation of inland lakes, and the 

 wear and tear of the land from these causes, which are still in continuous 

 action (as from the washing down of cliffs by the sea, and the formation of 

 mud deposits at the mouths of rivers), or the volcanic agencies which in 

 some places (as in Ireland) have cast up basalt over the chalk. 



There is a sort of transition formation which is classed with the 

 Cretaceous System, and termed " Maestricht," after the town in Belgium. It 

 appears that this is an upper chalk layer, an intermediary between the 

 Secondary and Tertiary, and here on the banks of the Meuse we find the 

 Mososaurus, the "lizard of the Meuse," of whose remains we give specimens 

 in the illustration. This transition chalk as we may call it to distinguish 

 it must have been laid down at a later period than the flinty chalk, and we 

 find it in many places. It serves therefore as a fitting introduction to the 

 Tertiary Period of Geological time. 



