276 DR. A. G-ILLIGAX OX THE PETROGRAPHY OF [vol. 1-XXV, 



Second Grit — A somewhat variable group of finely -grained sandstones and 



shales. 

 Shales. 

 Third Grit — A coarse massive gritstone between 200 and 300 feet thick, 



remarkable for its regular jointing and bold wall-like escarpments. 



Fine flaggy beds at the base, and a coal on the top frequently. 

 Shales. 

 Kinder scout Grit — Coarse, often conglomeratic, very massive, with 



occasional bands of flagstone. Where most largely developed, in two 



beds with shale between. Up to 500 feet thick. Also called Fourth 



Grit.' 



Marine bands are found at different horizons in the shales of the 

 whole series, perhaps the most notable in Yorkshire being that 

 exposed in Cayton Grill, north of Harrogate. When the excavations 

 were being made for the dam of the Eccup -Reservoir, Leeds, a 

 marine band was found yielding perhaps the richest fauna yet 

 discovered in the Millstone Grit, and I have described elsewhere 

 a series of five such marine bands passed through in a borehole 

 at the Tannery, Meanwood, Leeds. 1 



Coal-seams are also of frequent occurrence throughout the series, 

 but increase in number and thickness towards the top, and have in 

 many places been worked, though the coal is usually of inferior 

 quality. 



The subdivisions given above were found to apply well enough 

 as the work of surveying and mapping proceeded in Derbyshire and 

 South Yorkshire ; but, as it was carried farther north, some modi- 

 fication had to be introduced, as it was no longer possible to 

 identify with certainty the so-called Second and Third Grits, and it 

 was found better to apply the term ' Middle Grits ' to the large 

 number of separate grit-beds which occur between the Kinderscout 

 Grit and the Rough Rock. These upper and lower beds could still 

 be identified by their lithological characters and stratigraphical 

 position. 



VIII. Mode of Accumulation of the Mateeial. 



The assemblage of beds which make up the series is such as to 

 leave no reasonable doubt that the material represents the deltaic 

 deposits of some large river comparable in size with such a one as 

 the Mississippi, and doubtless, in the same way as that great river, 

 receiving tributaries from regions where climatic conditions were 

 of very different types. One has only to study the literature of 

 such deltas as those of the Mississippi, Ganges and Brahmaputra, 

 Irrawadi. or Nile, to realize the close resemblance between such 

 present-day deposits and the Millstone Grit of Yorkshire. Indeed, 

 one may go so far as to say that a study of our modern deltas is 

 absolutely essential to the understanding and correct interpretation 

 of the problems connected with the Millstone Grit and the 

 succeeding Coal Measures. It may be well to give an estimate of 

 the amount of material accumulated over the area of Yorkshire 



1 Trans. Leeds Geol. Assoc. Part xviii. (To be published.) 



