ROCKS. 69 



stoiies were evidently formed by a different process. 

 Here we have grains mechanically mixed, as in a layer 

 of sand on a sea-shore, and then in some way becoming 

 solid rock. The grains themselves may have been form- 

 ed at the outset when they were a part of some other 

 rock by a crystallizing process; but after the original, 

 rock became broken and worn in one way and another 

 into these grains, they became arranged by the agency 

 of water in layers, and changed into solid rock by means 

 which we shall hereafter consider. 



142. Rocks Composed of a Single Mineral. Rocks 

 sometimes are composed of one mineral alone. Pure 

 limestone, in its three varieties of common limestone, 

 chalk, and marble, is of this character, being simple car- 

 bonate of lime. Sometimes, though a rock may be a 

 single mineral throughout, that mineral may not be sim- 

 ple in its composition. Thus the rock called dolomite 

 is a magnesian carbonate of lime, or, in other words, 

 carbonate of lime and carbonate of magnesia intimately 

 combined, so as to make one mineral. Other examples 

 of rocks composed of a single mineral are quartz rock, 

 gypsum (sulphate of lime), serpentine, etc. 



143. Rocks Composed of more than one Mineral 

 Most rocks are constituted by a mixture of different 

 minerals. Granite is a familiar example, being a mix- 

 ture of quartz, feldspar, and mica. Sometimes the ingre- 

 dients are so finely mingled that, different from granite, 

 the composite minerals can not Jbe at all distinguished. 

 Pudding-stones are aggregates or conglomerates of peb- 

 bles, united by some cementing mineral, most commonly 

 silica, or oxyd of iron, or carbonate of lime. The peb- 

 bles may be of one kind or of different kinds, and they 

 vary much in size. When the conglomerate contains 

 angular pieces instead of rounded pebbles, it is called 

 breccia. Usually the pebbles or pieces are granite, or 

 quartz, or carbonate of lime, and the conglomerate is ac- 

 cordingly said to be respectively granitic, quartzose, or 



