ORE DEPOSITS. 



Most rock masses (from shrinking, in aqueous rocks ; and 

 cooling in igneous rocks) are divided into blocks, some- 

 times quite regularly, by means of what are called joints. 

 Deep in a mine, these joints fit closely ; not so at the sur- 



Fio. 13 AND 14. DKNUDATIOS OP STBATA. 



face. Most frequently the direction is at right angles to 

 the planes of bedding, In sandstones the joints are irre- 

 gular, and the blocks of different sizes; in limestone, the 

 joints are fewer than in shale and some kinds of slate, and 



Eio. 15. 



the blocks are generally cuboidal, the vertical joints being 

 very regular. 



The valuable minerals and metal-bearing deposits of the 

 earth are found as 



