374j Professor HofFiiiann 07i the Alhan Hills. 



from those of the Campagna di Koraa, by its greater solidity, 

 and by the freshness and lustre of its imbedded minerals. An- 

 gular foliag of mica, fragments of lava and slags, more rarely 

 augite, but above all, the leucite crystals, so abundant in the 

 Roman lavas, are heaped together in a light grey compact clay- 

 stone basis, which often encloses blocks of white marble, or 

 rather dolomite limestone, and fragments of rocks, consisting of 

 glassy felspar, and black or green mica, with grains of Hauyne, 

 exactly analogous to the matters which form the ejections of 

 Vesuvius, which we find enveloped in the same way in the Pepe^ 

 rino of Monte Somma. 



The peperino of the Alban Hills is always regularly stratified, 

 which, with the nature of its cement, go to prove its formation 

 having been influenced by the presence of water ; for it is not a 

 tufF formed by simple aggregation and subsequent cohesion 

 of its fragments, like what is so often found on Etna, but it 

 is a true product of aqueous deposition. Interposed between its 

 strata, we find here and there a few beds of slags, but more fre- 

 quently rough layers of basaltic lava, the cavities of which dis- 

 play the same variety of crystals of nepheline and mellilite, for 

 which the basaltic stream from the Tomb of Cecilia Metella, or 

 from the Capo di Bone near Rome, is already so remarkable. 

 But what is still more curiousis, that these alternations of peperino, 

 slags, and basalt, dip away with the utmost regularity from the 

 centre of this outer circus, their outgoings being distinctly seen on 

 the precipices of its interior ; presenting an exact picture of the 

 craters of elevation so beautifully distinguished by M. Von Buch. 

 If, then, the outer circle is the margin of the crater of elevation, 

 the interior one must necessarily be that of the crater of erup- 

 tion. This, accordingly, is the fact, but as it is the first extinct 

 crater which I had seen in Italy, I only arrived at this conclu- 

 sion after some little attention. For the margins of the cavity 

 are deficient in many points, and the inner walls of the crater 

 have a very slight inclination and are thickly covered with woods. 

 A portion of its western side is wanting altogether, and in the 

 interval rises the Hocca di Papa. The highest point of its mar- 

 gin is the long ridge of the Monte Cavo, about 2800 feet above 

 the level of the sea, and the bottom of the crater is about 750 

 feet lower, or 2000 above the sea. It is now covered with grass, 



