PART II. 



UTHOLOGICAL GEOLOGY. 



Lithological Geology treats of the materials in the earth's 

 structure : first, their constitution ; secondly, their arrangement or con- 

 dition. 



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-hundredth of 

 the earth's diameter. 



I. CONSTITUTION OF ROCKS. 



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 con- 

 solidated 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 of organic origin. 



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

 of plants or animals. Of this origin is the material 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 (if not too minute to require it), and 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 or- 

 ganic, the rest being of mineral origin. Such rocks usually contain 

 distinct remains of the shells or corals that have contributed to their 

 formation : these relics, whether of plants or animals, are called fossils 

 or organic remains, and the rocks are said to be fossiliferous. They 

 are also often called petrifactions, though not always really pet- 

 rified. 



