Mr. AlKlN on the Gravel at Litchfield. 427 



the appearances, which are the subject of the present notice, may be 

 most conveniently observed. 



The pebbles of granite, of syenite, and of greenstone, are in a state 

 of greater or less decomposition, but present nothing very remarkable, 

 those of schist are usually soft and rotten, have evidently swelled 

 since the period of their having been deposited here, in consequence 

 of which their lamina have parted from each other, and the interval 

 is not unfrequently filled by calcareous spar, which is occasionally 

 prolonged in the form of thin veins to the distance of a few inches 

 into the sand. The outer surface of the schist is but little changed, 

 but on breaking it, the interior is often found to be little else than a 

 black powder. 



The pebbles of quartz, which are numerous, appear to have un- 

 dergone no change whatever. 



The pebbles of limestone are the most abundant of all, and have 

 undergone considerable change. The madrepores and other coral- 

 loidal bodies of which the limestone is principally composed, resist 

 the solvent action of the water which percolates through the sand 

 much better than the compact calcareous matter that is interposed 

 between them does ; the same is the case with the slender veins of 

 calcareous spar by which the limestone is traversed ; hence in those 

 pebbles in which the process of disintegration is only moderately 

 advanced, the surface presents a corroded spungy appearance, the 

 prominent parts being composed of decaying madrepores and veins 

 of calcareous spar, while the interior of the mass is still compact 

 limestone. In other instances, where the decay has proceeded still 

 further, the whole of the interstitial matter is gone, and the madre- 

 pores themselves are reduced to a very tender friable mass. 



But the substances which have undergone the greatest change are 

 chalcedony and hornstone or chert. 



