380 Hejjorts and Proceedings — Geological Society of London. 



stone of an unusual character. For a distance of 1,600 yards from 

 the shaft this seam was good, and about 4 feet thick, A little 

 farther in, bands of stone from 1 to 10 inches thick made their 

 appearance in it, and, gradually increasing in thickness, these bands 

 eventually constituted the whole seam, the last traces of workable 

 coal disappearing at 250 yards from the point where the change 

 first began. The boundary of the barren area has been found for 

 a distance of 1,480 yards, and it runs north and south. The stone 

 is at first black, but after weathering it becomes grey, and displays 

 curious structures, among which are pisolitic or mammillated 

 structures, the intervening spaces being filled with coaly matter. 

 One specimen displays woody tissue filled with dolomite. Analyses 

 by Dr. W. Pollard yield from 18-5 to 13 per cent, of magnesia. 

 The phenomena are not those of a ' wash-out,' as there is no sign 

 of erosion, but there is proof that the dolomite was formed in almost 

 motionless water, and the conditions appear to have been those 

 under which a tufa would form. It appears to have been formed on 

 a spot to which clastic material scarcely gained access, and which 

 was reached even by vegetable matter in scant quantity and in 

 a finely divided condition. 



2. " On some Landslips in Boulder-clay near Scarborough." By 

 Horace W. Monckton, Esq., F.L.S., V.P.G.S. 



In 1893 Mr. Clement Reid drew attention to a foliated structure 

 developed in Drift at Beeston, near Cromer (Proc. Geol. Assoc, 

 vol. xiii, p. 66), and soon afterwards the present author noticed 

 examples of a very similar character in Bouldei'-clay on the 

 Yorkshire coast. The Clay forms much of the cliffs, and slips, 

 large and small, are very frequent. When the Clay is dry, vertical 

 cracks forming a sort of columnar structure occur, and the Clay 

 breaks away in lumps, while a moister condition causes flow, 

 producing more or less horizontal flow-structure which, as in the 

 Cromer case, has the appearance of irregular bedding. The author 

 illustrated his remarks by photographs of the cliffs taken by himself. 



IL— June 19th, 1901.— J. J. H. Teall, Esq., M.A., V.P.E.S., President, 

 in the Chair. The following communications were read : — 



1. " On the Use of a Geological Datum." By Beeby Thompson, 

 Esq., P.G.S., F.C.S. 



A proper interpretation of geological phenomena frequently 

 requires that allowance shall be made for differential earth-move- 

 ments that have taken place since the period under consideration. 

 Present differences of level in rocks of the same age may be due 

 to actual differences in depth of the sea-floor on which they were 

 deposited ; but they may also be the result of subsequent differential 

 earth-movements. The rock selected as a datum should combine 

 as far as possible the following characteristics : — It should be thin, 

 of considerable horizontal extension, having similarity in physical 

 characters and palaeontological contents over a large area, and 



