70 GEOLOGY. 



calcareous, this latter term being commonly applied to 

 all compositions in which there is limestone. 



144. Stratified and Unstratified Rocks. It is manifest 

 to the most superficial observer that some rocks are in 

 layers or strata, while others have a very different ar- 

 rangement. A full consideration of the modes of con- 

 struction of the different forms of rock belongs to an- 

 other part of this book. Suffice it to say now that the 

 materials of the stratified rocks were assorted and laid 

 down by water, and then by some means became solidi- 

 fied, while the unstratified were made under the influ- 

 ence of strong heat. 



145. Silicious, Argillaceous, and Calcareous Rocks. 

 Stratified rocks are divided into three classes, according 

 to their composition. The first class is the silicious or 

 arenaceous. This is the sandy division, called arenaceous 

 from arena, the Latin word for sand, and silicious, be- 

 cause the grains of which the rock is composed are 

 silica, or silex, as it is commonly called. The second 

 class, the argillaceous, are made of clay, which is a mix- 

 ture of silex and alumina, commonly in the proportion 

 of about three of the former to one of the latter. The 

 name comes from argil, a term applied technically to 

 alumina, and probably to clay. It is a characteristic of 

 rocks of this class that they give out a peculiar earthy 

 odor when breathed upon, which is not owing to the 

 presence of alumina simply, but to a combination of this 

 with some oxyd of iron. Calcareous rocks are those 

 which are composed of lime and carbonic acid. These 

 three classes of rocks are seldom found pure, but they 

 run into each other. For example, there are sandstones 

 which are not wholly silicious, the silicious grains being 

 united together by carbonate of lime. Whether any si- 

 licious or argillaceous rock has the carbonate of lime in 

 it can at once be ascertained by the application of sul- 

 phuric acid ( 60.) 



I pass now to consider some of the individual rocks. 



