1814.] Account of the Mountains of ancient Latium. 119 



they descend a little towards the lake. This I observed above the 

 cross at Nemi, and less distinctly above the monastery Palazzuola, 

 in that bed in which the consular fasces are engraved. 



It deserves attention that, on my way to the town Albano, not 

 far from tlie place where the Via Appia joins the present road, I 

 observed two opposite stratifications. The lower beds ascended 

 towards mount Albanus, and were cut off and covered by the 

 upper bed which rises towards the sea. 



I never observed the lava sperone stratified, although the rocks 

 of it are cut perpendicularly in the first Tusculan mount, from 

 which stones were taken to build the theatre and other houses. 

 The scoreaceous lava lying over it is divided into beds, as I have 

 already observed, usually parallel to the declivity of the mountain. 



I shall now notice only what appears to me most probable re- 

 specting the formation of these mountains. 



The peperino and volcanic tuff, from which it differs chiefly in 

 containing, instead of pumice, a porous clayey lava which is not 

 fibrous, and in containing numerous calcareous fragments, seem 

 to be ashes thrown out of a volcano, and afterwards conglutinated. 

 For under the improper name of ashes we may comprehend every 

 thing thrown out of a volcano in the form of powder. Whether 

 this powder be the real ashes of the fuel which feeds the volcano, 

 or only a mineral near the fire and by its agency reduced to powder; 

 whether the numerous crystals of augite and mica were formed by 

 the fire, or previously existed in the rocks acted on by the volcano; 

 whether the numerous fragments of basalt in the peperino had a 

 Neptunian origin, or are to be considered as pieces of ancient lava, 

 solidified under the earth, and thrown out in the form of fragments 

 by the agency of new heat : these are questions which cannot be 

 certainly decided from our present knowledge of volcanoes. It 

 seems more certain, that the limestone has not been formed by the 

 agency of fire, but only broken from the volcanic mountain, and 

 thrown out little altered. I think the same is the case with the 

 fragments of compound stones above mentioned, especially those 

 that contain haiiyne; although the mica and augite contained in 

 them indicate a certain relation to volcanic bodies. 



The craters, from which the peperino was thrown in the form of 

 powder, arc the lakes Albanus and Nemi, and the valley of Aricia. 

 This not only appears from the situation of these lakes in the centre 

 and highest part of the country, but likewise from this circum- 

 stance, that all the beds of peperino (those excepted which lie 

 towards mount Albanus) rise towards these lakes. 



It is not surprising that the same matter should be thrown out of 

 different craters : lor as long as the volcanic fire acts upon the same 

 materials, it must produce- the same bodies: nor can the crater at 

 which this matter issues make any difference. Thus for example, 

 the I'hlcgra-an field near Naples is composed of the same tuff, 



though we know thai it issued from a great variety of craters j 



nor can the tuff of the promontory of Miscnum be distinguished 



