Ch. XXXI.] 



PLIOCENE VOLCANOES. 



673 



The annihilation of the town may, perhaps, have been due to the 

 cavernous nature of the subjacent rocks ; for Catalonia is beyond the 

 line of those European earthquakes which have, within the period of 

 history, destroyed towns throughout extensive areas. 



As we have no historical records, then, to guide us in regard to the 

 extinct volcanoes, we must appeal to geological monuments. The an- 

 nexed diagram (fig. 723) will present to the reader, in a synoptical 

 form, the results obtained from numerous sections. 



Fiff. 723. 



Superposition of rocks in the volcanic district of Catalonia, 

 a. Sandstone and numttulitic limestone. c. Cones of scoria? and lava. 



5. Older alluvium without volcanic pebbles. 



d. IS ewer alluvium. 



The more modern alluvium (d) is partial, and has been formed by 

 the action of rivers and floods upon the lava ; whereas the older gravel 

 (6) was strewed over the country before the volcanic eruptions. In 

 neither "have any organic remains been discovered; so that we can 

 merely affirm, as yet, that the volcanoes broke out after the elevation 

 of some of the newest rocks of the nummulitic (Eocene) series of 

 Catalonia, and before the formation of an alluvium (d) of unknown 

 date. The integrity of the cones merely shows that the country has 

 not been agitated by violent earthquakes, or subjected to the action of 

 any great flood since their origin. 



East of Olotj'on the Catalonian coast, marine tertiary strata occur, 

 which, near Barcelona, attain the height of about 500 feet. From the 

 shells which I collected, these strata appear to correspond in age with 

 the Subapennine beds ; and it is not improbable that their upheaval 

 from beneath the sea took place during the period of volcanic erup- 

 tion round Olot. In that case these eruptions may have occurred 

 partly during the Xewer Pliocene, and partly during the Post-pliocene 

 period, but their exact age is at present uncertain. 



Older Pliocene Period. — Italy. — In Tuscany, as at Eadicofani, 

 Yiterbo, and Aquapendente, and in the Campagna di Roma, sub- 

 marine volcanic tuffs are interstratificd with the Older Pliocene strata 

 of the Subapennine hills in such a manner as to leave no doubt that 

 they were the products of eruptions which occurred when the shelly 

 marls and sands of the Subapennine hills were in the course of depo- 

 sition. This opinion I expressed* after my visit to Italy in 1828, 



* See first edition of Principles of Geology, vol. iii. chaps, xiii. and xiv., 1833 

 and former editions of this work, chap. xxxi. 

 43 



