174 Chemistry. * [Lecture SO. 



received a stroke -from a comet, which has thrown 

 all into confusion. Hutton supj)oses that the sea 

 is continually changing its bed, and is con- 

 stantly washing away the ground from one place 

 to another. 



Not to enter further into these hypotheses, 

 I shall only observe, that these strata are often 

 broken in different directions, in general perpen- 

 dicularly, so that the parts of the different strata 

 are separated from each other. The wideness of 

 these rents is different, sometimes a few inches, 

 sometime^ many yards. They are very com- 

 monly filled up with substances different from 

 the composition of the strata. You may have 

 observed in mountains the appearance of a white 

 stone, which passes through it like a vein. This 

 has been a rent filled up with a particular kind 

 of stone: these are also very common in the 

 strata of coal. When met with in the strata they 

 are called veins. They are generally of a consi- 

 derable hardness ; and in these the metallic sub- 

 stances are generally found. Where any of them 

 are not filled up with extraneous matter, the 

 internal surface is set with very beautiful and 

 regular crystals, called spar, projecting into the 

 cavity. 



1. The forms under which time, or calcareous 

 earth, appears, are various, but all agree in mild- 

 ness and insipidity, and hardly show any degree 

 of solubility in water: they all effervesce with 

 acids, and show the same kind of relation to 



