674 UPPER MIOCENE VOLCANOES. [Ch. XXXI. 



and it has recently (1850) been confirmed by the arguments adduced 

 by Sir R. Murcbison in favor of tbe submarine origin of tbe earlier 

 volcanic rocks of Italy.* These rocks are well known to rest con- 

 formably on the Subapennine marls, even as far south as Monte Mario 

 in the suburbs of Rome. On the exact age of the deposits of Monte 

 Mario new light has recently been thrown by a careful study of their 

 marine fossil shells, undertaken by MM. Rayneval, Yanden Hecke, and 

 Ponzi. They have compared no less than 160 species f with the shells 

 of the Coralline Crag of Suffolk, so well described by Mr. Seaiies 

 "Wood ; and the specific agreement between the British and Italian 

 fossils is so great, if we make due allowance for geographical distance 

 and the difference of latitude, that we can have little hesitation in 

 referring both to the same period or to the Older Pliocene of this 

 work. It is highly probable that, between the oldest trachytes of 

 Tuscany and the newest rocks in the neighborhood of Naples, a series 

 of volcanic products might be detected of every age from the Older 

 Pliocene to the historical epoch. 



VOLCANIC ROCKS OF THE UPPER MIOCENE PERIOD. 



Madeira and Porto Santo. — When treating generally of the origin 

 and structure of volcanic mountains, I have described (p. 646) at some 

 length the volcanic tuffs and other igneous rocks of Tertiary and Post- 

 tertiary date in the island of Madeira. Among the submarine de- 

 posits, it was stated that some were as old as the Upper Miocene 

 period, as shown by the fossil shells included in the tuffs which have 

 been upraised at San Vicente in the northern part of the island to the 

 heio-ht of 1300 feet above the level of "the sea. A similar formation 

 constitutes the fundamental portion of the neighboring island of Porto 

 Santo, forty miles distant from Madeira. The marine beds are there 

 elevated to an equal height, and covered, as in Madeira, with lavas of 

 supramarine origin. 



The largest number of fossils have been collected from tuffs and 

 conglomerates and some beds of limestone in the island of Baixo, off 

 the southern extremity of Porto Santo. They amount in this single 

 locality to more than sixty in number, of which about fifty are mol- 

 lusca, many of them in the state of casts only. 



Some of the shells probably lived on the spot in the intervals be- 

 tween eruptions ; some may have been cast up into the water or air 

 together with muddy ejections, and, falling down again, were de- 

 posited on the bottom of the sea. The hollows in some fragments of 

 vesicular lava, entering into the composition of the breccias and con- 

 glomerates, are partially filled with calc-sinter, being thus half con- 

 verted into amygdaloids. 



* Geol. Quart. Journ., vol. vi. p. 281. 



f Catalogue des Fossiles de Monte Mario, Eome, 1854. 



