BARIUM SULPHATE. 
49 
cementing material, which is barium sulphate. It seems 
probable that the protective cap of the pillar owes its 
comparative permanence against weathering action to the 
presence of a very large quantity of this almost insoluble 
sulphate. 
In some of the sandstone beds the barium sulphate is 
very unequally distributed, forming a network or a series 
of small masses more or less spherical in shape : in such 
sandstone the sand grains between the sulphate streaks and 
patches is quite loose, the result being that the weathered 
surface presents a honeycombed or mammillated appear¬ 
ance. In one bed which caps the Bramcote Hill the 
barium sulphate is present in little isolated patches about 
the size of a hazel-nut, and the weathering of this sand¬ 
stone accordingly yields little pebble-like masses of sand 
held together by the sulphate; this bed is accordingly usually 
described by the geologist as a pebble bed, although this 
name is not strictly appropriate. 
I have attempted to detect some evidence of the way in 
which this barium sulphate has been introduced into the 
original sand-bed. It may possibly have been deposited 
together with the sand, but if this is its origin it has 
certainly undergone physical change, since it exists now in 
a firm, compact, and crystalline condition. It seems certain, 
therefore, that it has either been originally deposited from 
aqueous solution, or has been rendered crystalline by the 
slow percolation of a solvent liquid through the sedimentary 
deposit, or else that it has been formed by the action of 
water containing calcium sulphate percolating through sand¬ 
stone originally cemented with barium carbonate. 
This double decomposition between calcium sulphate and 
barium carbonate has been artificially carried out by Biscliof, 
and the presence of calcium carbonate together with barium 
sulphate in some of the beds in question may indicate such 
an origin. With regard to the possibility of barium sulphate 
being deposited from solution, or being rendered crystalline 
by a solvent, it must be remembered that barium sulphate 
stalactites exist: the origin of these stalactites is undoubtedly 
similar to that of ordinary calcium carbonate stalactites, 
and one which I have recently examined consists wholly 
of the sulphate. I have also received sand-pebbles bound 
together with large and well-formed crystals of barium 
sulphate ; and that such crystals have been deposited from 
solution and not from fusion has been demonstrated by 
Biscliof almost beyond doubt. 
