CLASSIFICATION OF STRATIFIED ROCKS. 201 



As we have already stated, the gaps in the series are usually indi- 

 cated by unconformity. Xow, since unconformity always indicates 

 movements of the crust, changes of the outlines of sea and land, 

 changes of climate, and consequent changes in the fauna and flora, 

 these gaps mark the times of great revolutions in the earth's history, 

 and are therefore the natural boundaries of the eras, periods, etc. The 

 whole rock-series, therefore, is divided, by means of unconformity and 

 the character of the fossils, into larger groups called systems, and these 

 again into smaller groups called series and formations. The largest 

 groups are founded upon universal, or almost universal, unconformity, 

 and a consequent very great difference in character of organisms ; the 

 smaller groups are founded upon a less general unconformity and less 

 difference in character of the organisms. Corresponding with the great 

 divisions and subdivisions of the rock-system are the eras, ages, periods, 

 and epochs of the history. The several terms expressing the divisions 

 and subdivisions, both of the rocks and of the history, are unfortu- 

 nately used in a loose manner. We will try to use them in the manner 

 indicated. It will be observed that the divisions are founded upon (a) 

 unconformity, and (b) change in fossils. These generally accompany 

 each other, since they are produced by the same cause, viz., change of 

 physical geography. In some localities, however, they may be in dis- 

 cordance. In this case, the change of fossils is considered the more 

 important, and controls classification. 



CHAPTER III. 

 UNSTRATIFIED OR IGXEOUS ROCKS. 



Characteristics. — The uustratified are distinguished from the strati- 

 fied rocks, a, by the absence of true stratification — i. e., lamination of 

 sorted materials ; o, by absence of fossils ; c, by a more or less crystal- 

 line or else a glassy structure ; and, d, by their mode of occurrence ex- 

 plained below. 



General Origin. — They have consolidated from a fused or semi-fused 

 condition, and are, therefore, called igneous rocks. This origin is 

 shown by their structure ; by their occurrence in dikes and tortuous 

 veins ; by their effects on stratified rocks with which they come in con- 

 tact ; and by their resemblance in many cases to modern lavas. The 

 question of their probable mode of origin will be more specifically 

 treated after the description of their kinds. 



Mode of Occurrence. — Igneous rocks occur, a, underlying the strata, 

 and forming the great mass of the earth's interior; a', forming the 



