38 PRINCIPLES OF PALAEONTOLOGY. 



sometimes of regular layers ; * and there can be little doubt that 

 in many instances it has an organic origin, though this is not 

 capable of direct proof. When present, at any rate, in quantity, 

 and in the form of layers associated with stratified rocks, as is 

 often the case in the Laurentian formation, there can be little 

 hesitation in regarding it as of vegetable origin, and as an 

 altered coal. 



CHAPTER III. 



CHRONOLOGICAL SUCCESSION OF THE 

 FOSSILIFEROUS ROCKS. 



The physical geologist, who deals with rocks simply as rocks, 

 and who does not necessarily trouble himself about what fossils 

 they may contain, finds that the stratified deposits which form 

 so large a portion of the visible part of the earth's crust are 

 not promiscuously heaped together, but that they have a cer- 

 tain definite arrangement. In each country that he examines, 

 he finds that certain groups of strata lie above certain other 

 groups; and in comparing different countries with one another, 

 he finds that, in the main, the same groups of rocks are always 

 found in the same relative position to each other. It is pos- 

 sible, therefore, for the physical geologist to arrange the known 

 stratified rocks into a successive series of groups, or " forma- 

 tions, " having a certain definite order. The establishment of 

 this physical order amongst the rocks introduces, however, at 

 once the element of time, and the physical succession of the 

 strata can be converted directly into a historical or chronolog- 

 ical succession. This is obvious, when we reflect that any bed 

 or set of beds of sedimentary origin is clearly and necessarily 



* In the Huronian formation of Steel River, on the north shore of 

 Lake Superior, there exists a bed of carbonaceous matter which is regu- 

 larly interstratified with the surrounding rocks, and has a thickness of 

 from 30 to 40 feet. This bed is shown by chemical analysis to contain 

 about 50 per cent of carbon, partly in the form of graphite, partly in the 

 form of anthracite; and there can be little doubt but that it is really a 

 stratum of " metamorphic " coal. 



