Numerous illustrations of the intimate connection between the 
appearance or reappearance of organic forms and the existence of certain 
physical conditions are to be found in formations, partly of a fluviatile or 
estuarine, and partly of marine origin. The Carboniferous Limestone 
of Scotland furnishes instructive examples. This formation consists of 
three divisions. ‘The lowest of these contains some thick persistent 
bands of crinoidal limestone, which with their accompanying shales 
enclose an abundant marine fauna. The central group consists mainly of 
sandstones and shales, with numerous seams of coal and ironstone, 
With a maximum thickness of fully 1500 feet, it contains in abundance 
the remains of terrestrial vegetation, but the corals, crinoids, producti, 
spirifers, orthide, &., so profusely developed in the limestones below 
are entirely absent, while other forms (anthracosia, anthracomya, rhizodus, | 
“a 
¥ 
630 ‘PALHONTOLOGICAL GEOLOGY. — [Boox V. 
gyracanthus, &c.), either unknown or rare among the limestones, take © 
their place. It certainly might be thought that the older marine fauna 
had become extinct. Yet that this was not the case is proved by the 
reappearance of many of the old forms in an upper group of marine 
limestone forming the highest zone of the series. These organisms had 
been driven out of the area by a change of conditions, but as soon as 
these unfavourable conditions passed away, reappeared from some 
neighbouring region, where they had continued to live and suffer slight 
modification! 
* For further illustrations of the early appearance and long survival of species see 
postea, pp. 714, 716, 849, 
