LESSONS IN GEOLOGY. 325 



bituminous and anthracite. These rocks were formed from sedi- 

 ments that were deposited in layers or strata, and the rocks are 

 often called stratified rocks. 



Obsidian, propylite, trachyte, basalt and other volcanic prod- 

 ucts are called igneous rocks, and are unstratified. Doubtless 

 many of the granites and gneisses had their origin in trachyte or 

 basalt, and are strictly igneous rocks. 



Granite, syenite and other crystalline rocks are the last terms 

 in the series beginning with sediments, as well as in the series be- 

 ginning with obsidian or basalt. 



All rocks are divided by cracks or joints into irregular blocks 

 of various sizes and shapes. In stratified rocks the bedding 

 planes bound the blocks on two sides, and divisions nearly per- 

 pendicular to these and to each other bound the other sides. 

 The last are called joints. In the case of igneous rocks the 

 blocks are bounded on all sides by joints. These blocks vary in 

 form in different rocks ; in limestones they are regular and cu- 

 bical; in basalt regular and columnar; in slate, small and indis- 

 tinct ; in sandstone, large and irregular, etc. The cause of joints 

 is probably the shrinkage of the strata in consolidating frojn 

 sediments. Similar joints or cracks may be seen in the dried mud 

 of ponds that have been drained of water. Joints are confined 

 to individual strata, but fissures are fractures passing through 

 several or many strata, probably caused by movements of the 

 earth's crust. Fissures 30 to 50 feet wide and many miles in 

 length are known. Sometimes the rock on one side of a fissure 

 is forced upward so as to change the relations of the strata, 

 causing what is called a fault. In some cases the displacement is 

 as great as 15,000 or 20,000 feet. Fissures in many cases have 

 been filled with some rock-forming material, when they are known 

 as veins. Much of the gold and silver of the world came origi- 

 nally from veins. In some cases these fissures were filled by an 

 injection of igneous material from below; others are filled by mat- 

 ters deposited from water circulating through them. Joints and 

 fissures are geologically interesting, as they facilitate erosion, 

 determine the course of rivers and the location of valleys. 



