are formed almost wholly of Carbonate of Lime, some 

 of Silex, others partly of Oxide of Iron. I have some 

 of the second kind from a well dug in Richmond-park, 

 found among clay at the depth of 365 feet, taken notice 

 of by my friend Mr. J. Murray, then gardener to Mr. 

 Addington. I first received specimens of this nature 

 from Warwick by chance; and the appearance of it 

 was new to me, and to all whom I consulted about it. 

 Lady M. Thynne, who was going to Warwick, kindly pro- 

 posed to send me any mineralogical subject found in the 

 neighbourhood. I requested her Ladyship to pay some at- 

 tention to this, and was soon favoured with many curious 

 specimens of decomposing rock ; among which was that 

 here figured. It is a piece of rock, chiefly quartz and car- 

 bonate of lime, inclining to be somewhat spongy and 

 reddish with the oxide of iron : some of the cavities are 

 empty, others have some remains of the pebbles, and others 

 are nearly whole. The lower figure has the appearance of 

 having been a common pebble, the colouring part of which 

 has suffered oxvgenization so as to become a loose ferru- 

 ginous ochre, and the earthy parts of the stone are nearly se- 

 parated. Some of the best mill-stones are of this nature on 

 a larger scale, the quartz being left very porous j and these 

 are preferred for grinding of corn. 



