140 
ELEMENTS OF GEOLOGY. 
plained in Chapters V.,VI., and VII., has multiplied the ef¬ 
fects of condensation by pressure and cementation, and the 
modification produced by heat, fracture, contortion, upheaval, 
and denudation. 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 sub¬ 
stances. 
Why newer Groups should he studied first. —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 un¬ 
known 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 invertebrate, 
while some orders of animals and plants wholly unrejoresent- 
ed 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 in¬ 
creasing the remoter the era to which we refer, and the dif¬ 
ficulty of determining the true chronological relations of 
rocks is more and more enhanced, especially when we are 
comparing those which were formed simultaneously in very 
distant regions of the globe. Hence we advance with se¬ 
curer steps when we begin with the study of the geological 
records of later times, proceeding from the newer to the old¬ 
er, or from the more to the less known. 
In 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 short¬ 
est, 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. When, there¬ 
fore, we approach one of the newer groups before endeavor¬ 
ing to decipher the monuments of an older one, it is like en¬ 
deavoring 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. 
Nomenclature. —The origin of the terms Primary and Sec¬ 
ondary, and the synonymous terms Palaeozoic and Mesozoic, 
were explained in Chapter VHL, p. 123. 
The Tertiary or Cainozoic strata (see p. 123) were so call- 
