STRATIFIED ROCKS. 7 1 



origi)i, when speaking of the first appearance of a new type 

 of structure in the geological formations. It is difficult not 

 to associate some idea of causation with the terms origin and 

 originate, but the term initiation refers simply to an incoming 

 or a beginning to appear, leaving other questions open for 

 discussion. 



System. — This is the name for one of the larger geological 

 divisions, but there is no uniform rule for its application. 

 Originally, as proposed by Murchison, system was applied to 

 a series of rocks continuously exposed in some geographical 

 region. Thus, Silurian system was the series of rocks exposed 

 in Wales and western England at one time inhabited by the 

 Silures. The Devonian system was the series of rocks exposed 

 in south and north Devonshire ; Permian system, certain fos- 

 siliferous rocks first thoroughly studied in Perm, Russia; etc. 

 The term system was afterwards adopted as a name for a 

 large and prominent series of stratified rocks, as Carbonif- 

 erous system, Tertiary system, etc. 



Systems have been arbitrarily determined, and the list as 

 given, including those in which fossils have heretofore been 

 found, is as follows: Cambrian, Ordovician, Silurian, Devo- 

 nian, Carboniferous, Triassic, Jurassic, Cretaceous, Tertiary, 

 and Quaternary or Recent, or including Recent. These, as 

 has been said, are arbitrarily fixed, and there is in each case 

 a typical system expressed in the rocks of a particular region. 



These systems are applied with an approximate degree of 

 uniformity in all countries, although arbitrarily ; and era is 

 the time-designation which is applied to indicate the lapse of 

 time during the formation of the rocks of a single system. 



Geographical Conditions Determining the Local Characters of 

 Stratified Rocks. — There are a few particulars, regarding the 

 way in which these rocks were formed and their present 

 condition and order, which help to explain the conditions 

 under which the organisms lived in the past, and may ex- 

 plain why we have full records in some cases, very little rec- 

 ord in others, and in many cases very sparse and greatly 

 broken records of the life-histories we are seeking to read. 



The stratified rocks, as already stated, are the result of 

 water-action: First, erosion from already formed rocks; sec- 



