CHAP. IX.] JURASSIC PERIOD. 137 



It is rather remarkable that in three of these districts 

 the shore-beds should be chiefly limestones, and we must 

 infer that in these places, at least, very little detritus of any 

 kind was carried in from the land at the beginning of the 

 Liassic period. That this should be the case round the 

 Mendip island is quite natural, but that limestones should 

 be formed on the margin of the western inlet between 

 Wales and Devon requires explanation ; possibly this is to 

 be found in the supposition that freshwater lakes existed 

 in the country to the west, and that these for a time 

 arrested and detained the mechanical detritus brought 

 down by the rivers, leaving only the calcareous matter in 

 solution to be carried on to the sea by the effluent stream. 

 This, when added to the lime derived from the waste of 

 the Carboniferous Limestone along the shore, was more 

 than the sea-water could hold in solution, and the forma- 

 tion of limestones was the result. In the Scotch case, we 

 may suppose that the Sutherland basin was the lake or 

 lagoon which received the detritus, and thus allowed the 

 formation of limestones in the western gulf. 



The only shore-beds of Middle Liassic age preserved to 

 us are those on the Scottish coast, where the Scalpa beds 

 of Professor Judd, 1 in the islands of Scalpa, Skye, and 

 Eaasay, consist of calcareous sandstones, 200 feet thick, 

 containing the fossils of the English Marlstone. In Mull 

 they are represented by soft greenish sandstone with few 

 fossils ; and similar beds seem to have been formed in the 

 eastern basin, blocks of them occurring in the Boulder 

 clays of Elgin and Moray. 



No marginal deposits of Upper Liassic age are known, 

 but it may be noticed that round the Mendip Hills the 

 water was very shallow throughout the Liassic period, the 

 whole series being in some places represented by only 



1 " Quart. Journ. Geol. Soc.," vol. xxxir. p. 710. 



