694 EOCENE VOLCANIC ROCKS. [Ch. XXXIL 



nopsis, but they were not sufficient to enable me to determine -with 

 precision the age of the formation. 



There are many points in Auvergne where igneous rocks have been 

 forced by subsequent injection through clays and marly limestones, 

 in such a manner that the whole has become blended in one confused 

 aud brecciated mass, between which and the basalt there is sometimes 

 no very distinct line of demarcation. In the cavities of such mixed 

 rocks we often find chalcedony, and crystals of mesotype, stilbite, and 

 arragonite. To formations of this class may belong some of the 

 breccias immediately adjoining the dike in the hill of Gergovia; but 

 it cannot be contended that the volcanic sand and scorise interstratified 

 with the marls and limestones in the upper part of that hill were intro- 

 duced, like the dike, subsequently, by intrusion from below. They 

 must have been thrown down like sediment from water, and can only 

 have resulted from igneous action, which was going on contempo- 

 raneously with the deposition of the lacustrine strata. 



The reader will bear in mind that this conclusion agrees well with 

 the proofs, adverted to in the fifteenth chapter, of the abundance jof 

 silex, travertin, and gypsum precipitated when the upper lacustrine 

 strata were formed ; for these rocks are such as the waters of mineral 

 and thermal springs might generate. 



Eocene Volcanic Bocks. — The fissile limestone of Monte Bolca, 

 near Verona, has for many centuries been celebrated in Italy for the 

 number of perfect Ichthyolites which it contains. Agassiz has de- 

 scribed 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 natural history, for the sake 

 of the fossils. Had the lithographic stone of Solenhofen, now* re- 

 garded as so rich in fossils, been in like manner quarried solely for 

 scientific objects, it would have remained almost a sealed book to 

 palaeontologists, so sparsely are the organic remains scattered through 

 it. I visited Monte Bolca in company with Sir Roderick Murchison 

 in 1828, and w^e then satisfied ourselves that the fish-bearing strata 

 formed part of the Eocene rocks of the adjacent Vicentine : we also, 

 ascertained that the associated volcanic products, consisting chiefly 

 of peperino or brown basaltic tuff, were contemporaneous and inter- 

 stratified with marine deposits charged with the same fossils as those 

 which characterize the Middle Eocene group of Monte Bolca. In 

 some of the tuffs nummulites are met with, and two species, Num- 

 mulites globulus and JSf. mille-caput, were obtained by Sir R. Murchi- 

 son in a subsequent visit from beds intervening between those which 

 yield the chief supply of fossil fish. We observed dikes of basalt 

 cutting through vast masses of the peperino in Monte Postale, which 



* Murchison on the Structure of the Alps, Quart. Geol. Journ., vol. v. p. 225. 



