22 INTRODUCTION 



Industrial Geology includes mining and economic geology and is the 

 commercial application of geological principles. 



All of the above subjects are included in Part I of this volume, under 

 the head of Physical Geology. 



Historical Geology includes paleontology (Greek, palaios, ancient, 

 ontos, living being, and logos, discourse), or the study of the life of the 

 past as shown by its fossil remains ; or, in other words, fossil botany 

 and zoology. It also embraces p ale o geography (the geography of pre- 

 historic lands), which is concerned with the boundaries of the lands 

 and seas of the epochs and periods of the past, and with the evolution 

 of the continents. It also includes stratigraphy, or the arrangement 

 and succession of the strata, as indicated mainly by fossils. His- 

 torical geology calls to its aid all other branches of geology, in order 

 that the topography of the ancient land surfaces and the boundaries 

 of the lands and seas may be known, and that the climates to which the 

 earth was subjected may be determined. Such an exhaustive study 

 is necessary, since the causes of the rapid extinction of certain forms 

 of life, and of the sudden appearance and evolution of others, cannot 

 be known with certainty until the environment under which they 

 lived is learned. 



In general, it may be said that Historical Geology deals with the 

 evolution of the continents and of the life of the past. 



Length of Geological Time. — Without an appreciation of the 

 vastness of geological time (p. 417) as compared with the brief span 

 of a man's life, the work accomplished by the various geological 

 agents cannot be understood. This, conception of the length of 

 geological time can, perhaps, best be grasped by a comparison : 

 " Let a year be represented by a foot ; the average length of a human 

 life is then measured by the breadth of a dwelling house, and human 

 history is limited approximately to a mile; but the duration of 

 geologic time is comparable to the circumference of the globe." 



Present Status of Geology. — Much of the science of geology is 

 definitely known and has been learned as a result of the accurate 

 observation and careful reasoning of many geologists. It should, 

 however, be borne in mind that many of the theories are subject to 

 change, as will be pointed out from time to time in the following 

 chapters. This is due to the fact that geology deals with many 

 problems concerning which our knowledge is as yet incomplete, 

 notwithstanding careful observations and deductions. For example, 

 before it was known that the crust of the earth is heated, to some 



