104 LITHOLOGICAL GEOLOGY. 



(2.) Color, texture, and mineral composition. — This test may be 

 used to advantage within limited districts, yet only with caution. 

 There were at one time in geology an "old red sandstone" and a 

 " new red sandstone ; " and, whenever a red sandstone was found, it 

 was referred at once to one or the other. But now it is well under- 

 stood that color is of little consequence, except within a small geo- 

 graphical range. 



The same general remark holds with reference to mineral composi- 

 tion, as explained on page 102. 



One inference from the mineral constitution of a stratum is safe ; 

 that is, that a stratum is more recent than the rock from which its mate- 

 rial ivas derived. Hence, an imbedded fragment of some known rock 

 may afford important evidence with regard to the age of the contain- 

 ing stratum. 



The age of metamorphic and igneous rocks is sometimes judged of on lithological 

 evidence; but, with possibly some exceptions among Archasan metamorphic rocks, the 

 criterion is worthless. 



(3.) Fossils. — This criterion for determining the chronological order 

 of strata takes direct hold upon time, and, therefore, is very much the 

 best. Tlie life of the globe has changed with the progress of time. 

 Each epoch has had its peculiar species. Moreover, the succession of 

 life has followed a grand law of progress, involving under a single 

 system a closer and closer approximation in the species, as time moved 

 on, to those which now exist. It follows, therefore, that 



Identity of species of fossils proves approximate identity of age. 



Fossils are the best means we have for ascertaining the equivalency 

 of strata, or their identity of age. Equivalency is sometimes shown 

 in an identity of species ; more often in a parallel series of nearly re- 

 lated species ; often by an identity or close relation in the genera or 

 families ; often also in some j^rominent peculiarity of the various species 

 under a family or class. 



The progress in life has not consisted in change of species alone 

 The species of a genus often present, in successive periods, some new 

 feature ; or the higher groups under an order or class some modifica- 

 tion, or some new range of genera, so that, even when the species dif- 

 fer, the habit or general characters of the species, or the range of 

 genera or families represented, may serve to determine the era to 

 which a rock belongs, or at least to check off the eras to which it does 

 not belong. Thus Spirifer, a genus of mollusks, which has a narrow 

 form in the Silurian, has often a very broad form in the course of the 

 Devonian and the Carboniferous ages. Ganoid fishes, which have ver- 

 tebrated tails through loner ages, have their tails not vertebrated in 



