EOCENE VOLCANIC ROCKS. 
543 
But this idea has arisen from the intrusion ot the dike repre¬ 
sented in the annexed diagram (Fig. 604), which has altered 
the green and white marls both above and below. Never¬ 
theless, there is a real alternation of volcanic tuff with strata 
containing Lower Miocene fresh-water shells, among others 
a Melania allied to M. inquinata (Fig. 217, p. 268), with a 
Melanopsis and a Unio; there can, therefore, be no doubt 
that in Auvergne some volcanic explosions took place before 
the drainage of the lakes, and at a time when the Lower Mi¬ 
ocene species of animals and plants still flourished. 
Eocene Volcanic Rocks. —Monte Bolca. — The flssile lime¬ 
stone .of Monte Bolca, near Verona, has for many centuries 
been celebrated in Italy for the number of perfect Ichthyo- 
lites which it contains. Agassiz has described no less than 
133 species of fossil fish from this single deposit, and the 
multitude of individuals by which many of the species are 
represented is attested by the variety of specimens treasured 
up in the principal museums of Europe. They have been all 
obtained from quarries worked exclusively by lovers of nat¬ 
ural history, for the sake of the fossils. Had the lithograph¬ 
ic stone of Solenhofen, now regarded as so rich in fossils, 
been in like manner quarried solely for scientific objects, it 
would have remained almost a sealed book to palaeontolo¬ 
gists, so sparsely are the organic remains scattered through 
it. When I visited Monte Bolca, in company with Sir Rod¬ 
erick Murchison, in 1828, we ascertained that the fish-bearing 
beds were of Eocene date, containing well-known species of 
Nummulites, and that a long series of submarine volcanic 
eruptions, evidently contemporaneous, had produced beds of 
tuff, which are cut through by dikes of basalt. There is evi¬ 
dence here of a long series of submarine volcanic eruptions 
of Eocene date, and during some of them, as Sir R. Murchison 
has suggested, shoals of fish were probably destroyed by the 
evolution of heat, noxious gases, and tufaceous mud, just as 
happened when Graham’s Island was thrown up between Sic¬ 
ily and Africa in 1831, at which time the waters of the Med¬ 
iterranean were seen to be charged with red mud, and cov¬ 
ered with dead fish over a wide area.^ 
Associated with the marls and limestones of Monte Bolca 
are beds containing lignite and shale with numerous plants, 
which have been described by Unger and Massalongo, and 
referred by them to the Eocene period. I have already cited 
(p. 263) Professor Heer’s remark, that several of the species 
are common to Monte Bolca and the white clay of Alum Bay, 
a Middle Eocene deposit; and the same botanist dwells on 
* Principles of Geology, chap, xxvi., 9th ed., p. 432. 
