PART II. 



LITHOLOGICAL GEOLOGY. 



50. Lithological Geology treats of the materials in the earth's 

 structure : first, their constitution; secondly, their arrangement or condition. 



The earth's interior is open to direct investigation to a depth of 

 only fifteen or sixteen miles ; and hence the science is confined to 

 a thin crust of the sphere, sixteen miles being but one-five-hun- 

 dredth of the earth's diameter. 



I. CONSTITUTION OF ROCKS. 



51. Rocks. — A rock is any bed, layer, or mass of the material of 

 the earth's crust. The term, in common language, is restricted to 

 the consolidated material. But in Geology it is often applied to all 

 kinds, whether solid or uncompacted earth, so as to include, 

 besides granite, limestone, conglomerates, sandstone, clay -slates, 

 and the like solid rocks, gravel-beds, clay-beds, alluvium, and any 

 loose deposits, whenever arranged in regular layers or strata as 

 a result of natural causes. 



The constituents of rocks are minerals. But these mineral con- 

 stituents may be either of mineral or organic origin. 



(1.) The material of organic origin is that derived from the remains 

 of plants or animals. This is the fact with the mass of nearly all 

 the great limestone formations ; for the substance of the rock was 

 made from shells, corals, or crinoids, triturated into a calcareous 

 earth by the sea, and afterwards consolidated, just as corals are 

 now ground up and worked into great coral reef-rocks in the West 

 Indies and Pacific. In other cases only a small part of a rock is 

 organic, the rest being of mineral origin. Such rocks usually con- 

 tain distinct remains of the shells or corals that have contributed 

 to their formation : these relics, whether of plants or animals, are 

 called fossils, and the rocks are said to be f ossiferous . 



(2.) The material of mineral origin includes all that is not directly 



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