102 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 8, 



and the estuarine Coal-measures above, which characterizes the 

 south of England, is gradually lost in going northwards, — beds of 

 estuarine character gradually descending lower in the series in 

 Yorkshire, as first shown by Professor Phillips, still lower in Nor- 

 thumberland, as described by Mr. G. Tate ; while in Scotland, as has 

 been illustrated by Professor Geikie and other writers, the whole 

 Carboniferous series from top to bottom consists of estuarine 

 strata with subordinate marine beds intercalated. 



The estuarine strata of Jurassic age in Scotland exhibit two 

 different types of petrological character, which, though occasionally 

 passing into one another by insensible gradations, are usually very 

 distinct and easily recognizable. These we may distinguish as the 

 argillaceous and arenaceous types. The fossils of these estuarine 

 strata usually afford us comparative] y little aid in determining 

 the age of the several series : our knowledge of the succession of 

 forms among the freshwater mollusca is far too limited for us to 

 obtain much aid from this source ; and the few marine bands inter- 

 calated contain, as a rule, only specimens evidently dwarfed from 

 unfavourable conditions, and scarcely ever belonging to highly cha- 

 racteristic forms like the Cephalopods and Echinoderms. The 

 groups of marine strata, however, which alternate with the estua- 

 rine yield very fine and satisfactory faunas, by the study of which 

 we are able to fix with great precision the limits of age of the 

 latter. 



The arenaceous type of the estuarine strata is characterized by 

 beds of sandstone and grit, occasionally passing into conglomerates, 

 and becoming in places somewhat calcareous. These alternate in 

 some cases with subordinate beds of shale, and occasionally contain 

 thin and imperfect seams of coal. The sandstone strata, which often 

 attain to a great thickness, usually exhibit evidence of having been 

 deposited under comparatively shallow- water conditions : false- 

 bedding abounds ; surfaces with ripple-marks, worm-tracks, and 

 other indications of the proximity of the shore frequently occur ; 

 and the rock is usually crowded with fragments of carbonaceous 

 matter. Large masses of wood, sometimes preserved as jet, and at 

 other times presenting only hollow casts, abound in these sandstones ; 

 and occasionally vertical plant-markings, like those of the Lower 

 Oolite in England, are also found. Not unfrequently we observe a 

 rock made up of alternations of lamina? of sand and carbonaceous 

 matter, so exactly resembling the strata seen in sections of old sand 

 dunes, as strongly to suggest a similarity of origin. In all these 

 sandstone strata molluscan remains are usually extremely rare ; but 

 occasionally bands of obscure shells, almost always in the form of 

 casts, are found ; these sometimes belong to marine, and at other 

 times to freshwater genera. 



The general resemblance of all these strata of the arenaceous 

 type to those of the Lower Oolites of the Yorkshire coast is very 

 striking ; and it was not unnatural that, at the early date at which 

 they were first studied by Sir Eoderick Murchison, this should be 

 accepted as evidence of identity of age. Hence the whole of these 



