Earths. 189 



depend on the slow concretion of this sort of 

 matter, which has been softer, and, in conse- 

 quence of its moisture being evaporated, has 

 hardened and split, as we see clay or dry sand 

 after wet weather. What first led to this opi- 

 nion was an observation made upon the form 

 into which other bodies split in the same cir- 

 cumstances ; as starch, for instance. 



Pudding-stone and some kinds of free-stone 



^) 



may be referred to this class. 



Pudding-stone consists of gravel, sand, and 

 roundish pebbles cemented together, and has its 

 name from its appearance. 



Upon the whole, we find this stony matter in 

 a great variety of strata ; it is also found in the 

 veins of rocks in the form of quartz. This is a 

 flinty matter, which, when pure, is almost trans- 

 parent, strikes fire with steel, but is more brittle 

 and shivery than any of the other substances of 

 this kind. When it is very white, or has a 

 milky opacity, it still strikes fire with steel, but 

 is not so hard as common flint. This whiteness 

 and softness seem to depend on the admixture of 

 a gypseous spar ; it is therefore called the sparry 

 quartz. The black or gun flint is accidentally 

 found in the strata of other matter, always in 

 those of chalk or limestone : it is found in irre- 

 gular figures mostly lying in horizontal beds, at 

 no great distance from each other. 



Jasper is another appearance of this earth, in 

 the form of separate concretions. Chemists di- 



