138 
ELEMENTS OF GEOLOGY. 
formations in juxtaposition is almost invariably accompanied 
by a great dissimilarity in the species of organic remains. 
Frequent Unconformability of Strata. —Where the widest 
gaps appear in the sequence of the fossil forms, as between 
the Permian and Triassic rocks, or between the Cretaceous 
and Eocene, examples of such unconformability are very fre¬ 
quent. But they are also met with in some part or other of 
the world at the junction of almost all the other principal 
formations, and sometimes the subordinate divisions of any 
one of the leading groups may be found lying unconform- 
ably on another subordinate member of the same—the Up¬ 
per, for example, on the Lower Silurian, or the superior di¬ 
vision of the Old Red Sandstone on a lower member of the 
same, and so forth. Instances of such irregularities in the 
mode of succession of the strata are the more intelligible the 
more we extend our survey of the fossiliferous formations, 
for we are continually bringing to light deposits of interme¬ 
diate date, which have to be intercalated between those pre¬ 
viously known, and which reveal to us a long series of events, 
of which antecedently to such discoveries we had no knowl¬ 
edge. 
But while unconformability invariably bears testimony to 
a lapse of unrepresented time, the conformability of two sets 
of strata in contact by no means implies that the newer for¬ 
mation immediately succeeded the older one. It simply im¬ 
plies that the ancient rocks were subjected to no movements 
of such a nature as to tilt, bend, or break them before the 
more modern formation was superimposed. It does not show 
that the earth’s crust Avas motionless in the region in ques¬ 
tion, for there may have been a gradual sinking or rising, ex¬ 
tending uniformly over a large surface, and yet, during such 
movement, the stratified rocks may have retained their orig¬ 
inal horizontality of position. There may have been a con¬ 
version of a wide area from sea into land and from land into 
sea, and during these changes of level some strata may have 
been slowly removed by aqueous action, and after this new 
strata may be superimposed, differing perhaps in date by 
thousands of years or centuries, and yet resting conformably 
on the older set. There may even be a blending of the ma¬ 
terials constituting the older deposit with those of the new¬ 
er, so as to give rise to a passage in the mineral character of 
the one rock into the other as if there had been no break or 
interruption in the depositing process. 
Imperfection of the Eecord. —Although by the frequent dis¬ 
covery of new sets of intermediate strata the transition from 
one type of organic remains to another is becoming less and 
