as a Cementing Material of Sandstone. 3*75 



effected a separation of the sulphate from the sand by shaking the 

 powder about in water : the small cleavage flakes thus obtained gave 

 the optical characters of crystallised barium sulphate. Mr. Teall 

 further stated that the barium sulphate occurred in large irregular 

 crystalline patches, which included the sand grains ; the sand grains, 

 therefore, interrupted the reflections from the cleavage surfaces of the 

 barium sulphate, giving rise to the appearance generally known as 

 "lustre mottling" in petro-graphic literature. 



I had noted this irregular distribution of the barium sulphate. In 

 some parts of the rock the sulphate occurred in reticulated veins 

 inclosing small patches of more or less loose sand grains ; while in 

 other parts of the rock the sulphate occurred in spherical or oval 

 masses, between which looser sand was interspersed : occasionally, 

 however, the barium sulphate was uniformly distributed. 



The appearance presented by the weathered surface of the rock 

 varied much according to the mode in which the resistant sulphate was 

 distributed. When it was uniformly distributed, it formed an almost 

 complete protection against weathering : this was seen on the cap of 

 the great pillar of this rock, which is locally known as the " Hemlock 

 Stone." The reticulated distribution of the sulphate caused the surface 

 of the weathered rock to present a fretted surface, with the thin veins 

 of sulphate projecting from the surface. "When the sulphate had bound 

 together spherical or oval masses in the substance of the sand, these 

 were left in pebble-like forms as soon as the loose sand had been 

 washed out from between them ; and the resulting layers of loose sand, 

 inclosing the rounded masses of sand bound together by the sulphate, 

 had been not unnaturally classed by the geologists who had visited 

 the district, as pebble-beds. 



In discussing various ways in which this barium sulphate might 

 have been deposited, I drew attention to the frequent deposition of 

 barium sulphate from colliery water in the neighbourhood of Newcastle- 

 upon-Tyne, and described some of these deposits.* And I further pointed 

 out that Dr. Bedson had shownf that barium chloride was a common 

 constituent of the colliery waters in the district in which these barium 

 sulphate deposits occur : and that it was present to the extent of 137*2 

 parts per 100,000 in some of these waters. It was evidently only neces- 

 sary that water containing sulphuric acid, or a soluble sulphate should 

 mingle with the barium chloride water in order to explain the deposi- 

 tion of barium sulphate in the positions in which it was found. In 

 colliery districts a frequent source of ferrous sulphate and of sulphuric 

 acid is found in the iron pyrites in the beds of coal and shale. And I 

 suggested that the constant occurrence of fine veins of calcium sulphate 

 throughout the sandstone of the Nottingham district would account 



* ' Roy. Soc. Proc.,' June, 1889. 

 t M.S.C.T.,' vol. 6, p. 712. 

 VOL. LXIV. 2 G 



