500 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. | 
similar. Hence mere resemblance in mineral aspect usually cannot 
be regarded as satisfactory evidence of contemporaneity except 
within comparatively contracted areas. ‘he Carboniferous Lime- 
stone has already (p. 494) been cited as a notable example. Typi- 
eally in Belgium, Central England, and Ireland, it is a thick cal- 
careous group of rocks, full of corals, crinoids, and other organisms, 
which bear witness to the formation of these rocks in the open sea. 
But traced into the north of England and Scotland, it passes into sand- 
stones and shales, with numerous coal-seams, and only a few thin 
beds of limestone. The soft clay beneath the city of London is re- 
presented in the Alps by hard schists and contorted limestones. We 
conclude therefore that lithological agreement, when pushed too far, 
is apt to mislead us, partly because contemporaneous strata often 
vary greatly in lithological character, and partly because the same 
lithological characters may appear again and again in different ages. 
By trusting too implicity to this kind of evidence, we may be led to 
elass together rocks belonging to very different geological periods, 
and on the other hand to separate groups which really, in spite of 
their seeming distinction, were formed contemporaneously. | 
2. It is by the remains of plants and animals imbedded among 
the stratified rocks that the most satisfactory subdivisions of the 
geological record can be made, as will be more fully stated in Books 
V. and VI. A chronological succession of organic forms can be - 
made out among the rocks of the earth’s crust. A certain common 
facies or type of fossils is found to characterize particular groups of 
rock, and to hold true even though the lithological constitution of 
the strata should greatly vary. Moreover, though comparatively few 
species are universally diffused, they possess remarkable persistence 
over wide areas, and even when they are replaced by others, the same 
general facies of fossils remains. Hence the stratified formations of 
two countries geographically distant, and having little or no litholo- 
gical resemblance to each other, may be compared and paralleled 
simply by means of their enclosed organic remains. 
Order of Superposition—the Foundation of Geological 
Chronology.—As sedimentary strata were laid down upon one another | 
in a more or less nearly horizontal position, the underlying beds must 
be older than those which cover them. ‘This simple and obvious 
truth is termed the law of superposition. It furnishes the means of 
determining the chronology of rocks, and though other methods of 
ascertaining this point are employed, they must all be based 
originally upon the observed order of superposition. The only case 
where the apparent superposition may be deceptive is where the strata 
have been inverted, as in the examples cited from the Alps 
(pp. 314, 518), where the rocks composing huge mountain masses 
have been so completely overturned that the highest beds appear 
as if regularly covered by others which ought properly to underlie 
them. But these are exceptional occurrences, wherein the true 
order can usually be made out from other sources of evidence. 
