CLASSES OF ROCKS 



261 



definite layer structure. A cross section, especially when 

 polished, shows multitudes of parallel lines. These rocks 

 are of different materials, a layer of one material covering 

 a layer of another material. Thus, the rock under the 

 Niagara River, at the Falls, is a hard limestone (Fig. 

 225). This rests upon a layer of soft shale. Sandstone 

 also forms one of the layers. 



Each set of layers of rock of one kind is called a stratum. 

 The plural is strata. Rocks made up of strata are called 

 stratified rocks (cf. 132, 

 Fig. 107). They were 

 deposited, as sediment, 

 under water; hence they 

 are also called aqueous 

 rocks, and sedimentary 

 rocks. Limestone, sand- 

 stone, shale, and con- 

 glomerate are stratified 

 rocks. 



Granite does not have 

 this layer structure, and 

 belongs to the class of 

 unstratified rocks. The 

 minerals present in gran- 

 ite are crystallized in 

 separate masses, and the 

 masses are closely interlaced. This is what we should 

 expect if quartz, feldspar, etc., had been melted together, 

 and then allowed to cool very slowly. 



Other common unstratified rocks are basalt, lava, and pumice. 

 Basalt (Fig. 226) is often called ' ' trap rock." Lava and pumice have a 



Fig. 226. 



Giant's Causeway, Ireland; Made of Basalt. 



Copyright, The International Stereograph 



Co., Decatur, 111. 



