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104. — On the Microscopical Structure of Hearthstone from 

 Betchworth, Surrey. 



By W. Murton Holmes. 



(Bead September 14th, 1892.) 



Immediately overlying the beds of gault clay at Betchworth is 

 a layer of rock some ten feet in thickness, belonging to the upper 

 greeusand formation, known as hearthstone. 



The underground workings for this hearthstone were visited 

 by several members of this Club on August 1st. They extend 

 for some little distance into the escarpment, from the summit of 

 which, on a clear day, there is a magnificent view across the 

 Weald. 



The hearthstone, when first quarried, is of a dull greenish 

 colour, and soft earthy consistence, easily crumbling between the 

 finger and thumb. When dry, it is of a greyish or buff colour, 

 and somewhat harder. Treated with hydrochloric acid, a por- 

 tion is dissolved with effervescence, indicating the presence of 

 calcareous matter. 



Examined with a lens, it shows numerous dark specks in a 

 light-coloured matrix, with here and there a glittering particle 

 of mica. In order to examine it more conveniently, a small 

 lump was carefully disintegrated under water, and, by means of 

 fine gauze and decantation, was divided into five portions, so 

 that the particles were of tolerably uniform size. 



The first and second portions consist of the coarser particles, 

 and are of a buff colour. The third portion is much darker in 

 colour, and contains the great bulk of the dark particles pre- 

 viously mentioned. The fourth and fifth are much paler in 

 colour, and consist of the finest particles. Assuming the bulk of 

 the first portion, after being allowed to settle, to be represented 

 by 1, that of the second would be about 2h, that of the third 5, 

 that of the fourth 5, and that of the fifth and finest 2. 



These figm-es are fairly approximate, but it would have been 

 more accurate to have dried and weighed the various portions. 

 The coarsest portion consists principally of particles of mica, 

 some colourless, some brown, and others of a greenish tint. 

 Foraminifera are also present in fair abundance, and some in a 

 good state of preservation. They belong to the genera Globi- 

 gerina, Textularia, Pnlviindina, and Eotalia. Casts of the same 

 in glauconite are also present. The remaining particles consist 

 of fragments of sponge spicules, m many cases showing clearly 

 the central canal, which is occasionally filled with glauconite. 

 The fragments are frequently covered with spherical or disk-like 

 particles of silica. With the exception of the mica, most of the 



