426 DESCRIPTIONS OF ROCKS. 



55 per cent, or more of silica, and the basic usually less 

 than 52. 



6. The feldspars are divided, according to their bases, 

 into (1) potash-feldspars, including orthoclase and micro- 

 cline; and (2) those which may be designated soda-lime- 

 feldspars, namely the species albite, oligoclase, andesite, 

 labradorite, and anorthite, which yield either soda, or lime, 

 or both, on analysis. The term plagioclase has been used 

 for the latter ; but it is no longer applicable since microcline 

 is plagioclase. Under the heading potash-feldspars, as used 

 beyond, leucite also is included ; and under that of soda- 

 lime-feldspars, nephelite and sodalite, and also the minerals 

 of the saussurite group. 



The kinds of rocks are described under the heads of — 



1. Fragmental Bocks, exclusive of Limestones. 



2. Limestones, or Calcareous Rocks. 



3. Crystalline Eocks, exclusive of Limestones. 



No strongly denned limit exists between the fragmental 

 and crystalline rocks. But still they are for the most part 

 widely diverse in character and aspect. 



In the names of rocks, the termination ite is here changed to yte, as 

 done in the author's " System of Mineralogy" (1868), in order to dis- 

 tinguish them from the names of minerals. Granite is excepted. 



I. Fragmental Rocks, exclusive of Limestones. 



1. Conglomerate. — A rock made up of pebbles or of coarse 

 angular fragments of rocks of any kind, (a) If the pebbles 

 are rounded, the conglomerate is a pudding-stone ; (J?) if 

 angular, a breccia. 



Conglomerates are named according to their constituents, siliceous 

 or quartzose, granitic, calcareous, porphyrinic, pumiceous, etc. 



2. Grit.— A hard, gritty rock, consisting of coarse sand, 

 or sand and small pebbles, called also millstone grit, be- 

 cause used sometimes for millstones. 



3. Sandstone. — A rock made from sand : a consolidated 

 sand-bed. 



Varieties.— a. Siliceous or Quartzose; consisting chiefly of quartz, 

 b. Granitic; made of granitic material or comminuted granite, c. Mi- 

 caceous ; containing much mica. d. Argillaceous ; containing much 

 clay with the sand. e. Gritty ; hard and containing small quartz peb- 



