TRANSACTIONS OF SECTION C. 329 



and gigantic fish of Burdie House are all indications of terrestrial conditions. 

 All these evidences of Carboniferous vegetation occur in the geological horizon of 

 the Carboniferous limestone and Yoredale series. 



Never entirely free from sandy impurities the Carboniferous Limestones north 

 of the central barrier gradually became covered with a thick arenaceous series 

 containing here and there marine fossils and traces of coal plants. Those on. 

 the Yoredale strata consist mainly of the sediments of a somewhat distant 

 north-westerly land, the plants of which were carried to sea by rivers and depo- 

 sited here and there on the sea floor. It would appear from the evidence collected 

 by the Geological Survey that, after a very considerable thickness of these rocks 

 had collected, either a filling up of the shallow sea or a slight upheaval of the floor 

 occurred, for denudation of their surface happened, considerable depressions and 

 ridges being produced on it. On those spaces and ridges, and indeed on the whole 

 surface of the Yoredale rocks, collected strata which are popularly called the millstone 

 grits, so well seen west of Sheffield. All the depths of this great land wreckage, 

 consisting of silicious and felspathic sandstones' and shales, accumulated on a sink- 

 ing area, some near land and the rest in deeper places. And here and there coal 

 seams are found intercalated, being evidences of the existence of contemporaneous 

 vegetation. Some of them are workable, and others are only valuable as evidences 

 of the existence of the vegetation of the age ; many are placed on a hard silicious 

 or ganister bed, but some have an underlying fire-clay. They are very usually 

 covered with deposits ^containing goniatites and aviculopecten, which doubtless 

 are the remains of marine organisms. 



Admitting, therefore, that some of this millstone grit coal may be the result of 

 the drifting and sinking of the vegetation from off lands rather remotely situated, 

 it is still evident, from the existence of the under-clays elsewhere, that some of the 

 grits, by silting up, or by slight upheaval, above sea level, formed the subsoil of 

 swampy ground on which vegetation grew. This approach of the millstone grit 

 sea floor to above sea level was decided enough in the region of the great coal-field 

 around us, for a conglomeratic rock — the rough rock — occupies a somewhat defi- 

 nite horizon on the top of the series. 



This rough rock collected in shallow water, and it is important to the geolo- 

 gical surveyor, for it formed the base on which the coal measures proper rest ; 

 and it is suggestive to the physical geologist that a general and wide, but not great, 

 upheaval took place which removed the ocean of the day further off, and which 

 determined a total change in the direction of sediment-depositing currents. 



Hitherto the greatest thickness of the sediments of the millstone grit age had 

 been towards the north-west, and the direction of the currents had been from 

 north-west to south-east, but subsequently, as has been suggested from very strong 

 evidence by Sorby, the depositing currents of the next age had no very definite 

 direction. But the Carboniferous land of this part of Europe was not yet remote 

 from the sea. Much of it was on the borders of estuaries, and the aspect of nature 

 was probably that of wide flats of grit covered usually by terrestrial vegetation and 

 occasionally overwhelmed by sea. In fact, both practically and theoretically there is 

 much difficulty in separating the millstone grits from the lower coal measures. The 

 lower measures contain some thick and widely-spread sandstones, and the important 

 coal seams, in some instances rest on a hard ganister bed, and in others on a fire- 

 clay. And to add to the similarity of the deposits of the upper grits and lower coal 

 measures, marine fossils, such as species of goniatites, aviculopecten, and posido- 

 nomya, are intercalated above the coals. But the evidences of marine invasion 

 ceased as the deposits accumulated, and more perfect terrestrial conditions arose. The 

 Elland flag-stones, for instance, such prominent features to the west of this town 

 and in the neighbourhood of Halifax, are fresh-water deposits, and are undoubtedly 

 accumulated in an under-clay indicative of terrestrial conditions. 



In the region north of the northern barrier successive coal seams and impure 

 limestones and fire-clays occurred during the age of the depositions of the English 

 grits, and then a thick fossiliferous sandstone was followed by the upper coals of 

 Mid-Lothian. 



All the minor upheavals and upsiltings of this long age were subordinate to a 



