131 



THE PHYSIOGRAPHY OF NORTH BRITAIN IN 

 MILLSTONE GRIT TIMES. 



W. S. BISAT. 



The publication of Dr. Gilligan's paper on the petrology 

 of the Millstone Grits {Q.J.G.S., Vol. LXXV., p. 251 et. seq.) 

 brings up-to-date the pioneer work of Sorby in this field of 

 research, and throws a flood of light on the occurrence and 

 nature of the heavy minerals contained in the coarse grit beds. 

 The large pebbles from the Middle Grits of the Silsden area 

 are also most interesting, and are well illustrated by fine 

 micro -photographs. It is to be hoped that other areas will 

 also yield pebbles of like interest. 



The restoration of the deltaic area of Grit times (illus- 

 trated by a map), is based on the petrological evidence, 

 supported by current bedding. The coast lines shewn are 

 in the main essentials the same as those adopted by Jukes 

 Browne for the Lower Carboniferous (' The Building of the 

 British Isles,' 3rd Edition (1911), Fig. 23), except that the 

 map follows Roberts (' Introduction to Modern Geology, 1893') 

 in omitting land to the east, and in its place is shewn a large 

 river descending from the north through a considerable 

 distance of what is apparently water. 



The absence of a drainage system from the land to the 

 north-west, and the acute angle the river to the east makes 

 with the watershed, both look improbable, and it appears 

 to the writer that the evidence for the Millstone Grits having 

 entered our area from the north-east is not wholly satisfactory. 



As stated above, the coast lines shewn are practically 

 identical with those of Lower Carboniferous times. This 

 hardly looks as if sufficient importance had been attached ta 

 the general regional uplift which ushered in the Millstone 

 Grit period. This uplift must have had the effect of consider- 

 ably reducing the area under water and bringing the north- 

 west coast line of Lower Carboniferous times well down into 

 Yorkshire. It is indeed quite probable that a land bridge 

 across Ireland connected the north-west land with 'St. George's 

 Land,' for Coal Measures rest directly on Carboniferous 

 Limestone, near Kiltamagh in Mayo, (Jukes Browne, op. cit., 

 p. 177). There may, however, possibly always have been a 

 narrow channel connecting the southern sea with the northern 

 area, for the fauna of the marine bands intercalated in the 

 Grits is of the southern (Pendleside) character. The only 

 marine band containing the northern (Yoredale) fauna is 

 the Cayton Gill band, and that is confined to the north- 

 eastern corner of the Grit exposures. 



Evidences of a ' non-sequence,' suggesting land in York- 



1921 April 1 



