181 1.] Accovnl of the Mountains of ancient Lalium. 121 



covered by the porous. The former is visible but seldom on the 

 higher parts of mountains and only in small streams ; but at the 

 bottoms of the mountains we fir.d it in greater abundance consti- 

 tuting hills. Over the lava sperone lie beds composed of frag- 

 ments of the same lava, conglutinated together; over them lie 

 beds of scoriae, or fragments of sperone, more porous and vitrified, 

 and not conglutinated together; over the whole is deposited a dark 

 red fatty clay, which seems to owe its origin to the ashes thrown 

 out towards the end of the eruption. 



The beds of peperino descend towards the lake on that side 

 which looks towards mount Albanus, because they were lifted out 

 of their original position by the strength of the fire which threw 

 out the lava of mount Albanus. 



I could no where see distinctly the lava of mount Albanus lying 

 over the peperino; but you can sec thick beds of scoriae, doubtless 

 produced at the same time with the lava, covering the peperino 

 above the lake Nemi. In the same manner, not far from the 

 margin of the lake Albanus, above the monastery called Palazzuola, 

 the scoriae of mount Albanus begin to predominate, and the lava 

 itself succeeds them towards the Rocca di Papa. If you suppose 

 the peperino formed after the lava, it is impossible to explain why 

 we can find no traces of peperino or ashes on mount Albanus or the 

 Tusculan mounts, though these mountains are so near the craters of 

 the peperino, that they may be said to rise from the very margin of 

 them. Vou Buch, indeed, observed basalt lying under the pepe- 

 rino, in a valley near the fountain called Dell Aqua Tepidula. I 

 did not see the place. But as the lava, though formed after the 

 peperino, may have passed through its fissures, it may no doubt 

 be seen lying under it, though of later formation. 



The origin of the compact lava surrounding the walls of pepe- 

 rino, and in many places covering it, is of more difficult explana- 

 tion. Two opinions respecting it may be entertained. Either the 

 craters constituting at present the two lakes, after they had thrown 

 out the peperino, became also filled with lava which in some parts 

 adhered to the walls of peperino, and when the volcanic energy 

 was at an end falling partly back again into the abyss, left an open- 

 ing as a receptacle for waters. Or all this lava flowing down from a 

 higher crater, and particularly from mount Albanus, adhered to 

 the walls of the peperino. What makes against this latter opinion 

 i-, that the lava surrounds the lake even in the parts most remote 

 from mount Albanus. Thus the lake Albanus is surrounded with 

 of lava between Castle Gandolfo and the town Albaho, 

 The Bame remark applies to the lake Nemi, on its south side, be- 

 tween the towns of (iensano and Ncmi. Nor in these places is 

 any lava to be seen lying above the peperino. 



On lb.it account, 1 am disposed to believe thai the compact lava 



coin- from the same crater with the peperino. I conceive that 



only the lava sperone, whieh forms a rock below Nemi, flowed 

 from mount Albanus, as the scoria- of that mountain extend as lat- 

 in of the lake. To confirm this opinion, it would be 



