STRATIFIED ROCKS. /7 



tion of rocks. There are certain rocks that are relatively 

 primitive ; these are crystallized and compact, as granites and 

 gneisses. There are other rocks that are of sedimentary ori- 

 gin ; these are secondary in original formation ; they are made 

 of fragments of rocks, and are in stratified form, and lie upon 

 primitive rocks whenever the two are in contact. There are 

 still other geological formations that generally are not in com- 

 pact form, but are composed of loose fragments, sand, and 

 fine mud, or soil, and naturally lie above the others. 



A second law established by experience is that (with ex- 

 ception explained by later disturbance) for the sedimentary 

 rocks natural order of superposition indicates relative chrono- 

 logical order of formation ; viz., in any given case of two 

 stratified rocks the underlying rock is the more ancient. 



A third law is that the mineral character of any particular 

 stratified rock bears no necessary relation to its age. As, for 

 instance, rocks of the same composition, structure, and color, 

 but coming from separate geographical regions, may be of 

 entirely different geological ages. 



Fourth. It is an established law that there is some definite 

 relationship between the characters of the fossils and the rela- 

 tive geological age of the rocks in which they occur. This law 

 is formulated in the classification Paleozoic, Mesozoic, Ceno- 

 zoic, applied to the respective geological formations in their 

 chronological order. 



In accordance with these laws a classification of forma- 

 tions has been formed (Cambrian, Ordovician, Silurian, De- 

 vonian, etc.) in which the relative antiquity of the systems is 

 expressed. This constitutes the formation-scale, and it is 

 based upon the series of strata, lying one upon another, com- 

 posed of sedimentary materials of various kinds forming sand- 

 stone, limestone, shales, and conglomerites, etc., originally 

 nearly horizontal in position, but now variously tilted and 

 folded. In such rocks the fossils are found from which the 

 time-scale proper is constructed. The recognizable units of 

 this time-scale are the periods, characterized by fossil fauna- 

 floras, whose characteristic species may be distinguished the 

 world over and thus form the marks of the standard time- 

 scale for the study of the history of organisms. 



