€h. XIII.] OF TERTIARY FORMATIONS. 1§1 



posits have undergone. Masses, for example, which were originally 

 soft and yielding have been condensed by pressure, others which were 

 incoherent have been solidified by the infiltration of mineral matter 

 which has cemented together their separate parts ; others have been 

 modified by heat, traversed by shrinkage cracks, and partially crys- 

 tallized, or the strata have been fractured during earthquakes, or bent 

 and contorted by lateral pressure, or thrown into a vertical position, or 

 even overturned so that the original order of superposition has been 

 inverted, and the beds which were at first the lowest have become the 

 uppermost. 



The organic remains also have sometimes been obliterated entirely, 

 or the mineral matter of which they were composed has been removed 

 and replaced by other substances, as when calcareous fossils have been 

 silicified. 



We likewise observe that the older the rocks the more widely do 

 their organic remains depart from the types of the living creation. 

 First, we find in the newer tertiary rocks a few species which no longer 

 exist, mixed with many living ones, and then, as we go farther back, 

 many genera and families at present unknown make their appearance, 

 until we come to strata in which. the fossil relics of existing species are 

 nowhere to be detected, except a few of the lowest forms of inverte- 

 brate, while some orders of animals and plants wholly unrepresented 

 in the living world begin to be conspicuous. 



When we study, therefore, the geological records of the earth and 

 its inhabitants, we find, as in human history, the defectiveness and 

 obscurity of the monuments always increasing the remoter the era to 

 which we refer. The difficulty of determining the true chronological 

 relations of rocks is also more and more enhanced, especially when 

 we are comparing those which were formed simultaneously in very dis- 

 tant regions of the globe. Hence we advance with securer steps when 

 we begin with the study of the geological records of later times, 

 proceeding from the newer to the older, or from the more to the less 

 known. 



*[n thus inverting what might at first seem to be the more natural 

 order of historical research, we must bear in mind that each of the 

 periods above enumerated, even the shortest, such as the Post-tertiary, 

 or the Pliocene, Miocene, or Eocene, embrace a succession of events 

 of vast extent, so that to give a satisfactory account of what we already 

 know of any one of them would require many volumes of the size of 

 this treatise. When, therefore, we approach one of the newer groups 

 before endeavoring to decipher the monuments of an older one, it is 

 like endeavoring to master the history of our own country and that 

 of some contemporary nations, before we enter upon Roman History, 

 or like investigating the annals of Ancient Italy and Greece before we 

 approach those of Egypt and Assyria, That there are inconveniences 

 in thus inverting the order in which the successive events are spoken 

 of I fully admit, but there are also unquestionable advantages, and 



