2G8 Notice of Active and Extinct Volcanos. 



varieties of volcanic substances, intermixed with a white sand, 

 but never agglutinated so as to form a tuff. Its form is that of a 

 compressed or oblong cone, and it has in its centre a crater al- 

 most as deep as the mountain is lofty. 



" Near the bottom of the crater are one or two small caverns, 

 the interior of which I found covered here and there with an 

 efflorescence, having an alkaline taste. The sand near the foot 

 of the mountain, even under the sea, possesses so high a temper- 

 ature when brought up from a little below the surface of the wa- 

 ter, that we are led to conclude that the volcanic action is still 

 going on to a certain extent, and the same inference may be 

 drawn from the extreme heat of the water which gushes from 

 the rock in a cavern not far distant, called the Baths of Nero, 

 which is sufficient in a very few minutes to boil an egg.'''' 



Solfatara. — The celebrated hill of Solfatara is an extinct 

 or dormant volcano ; the rock is a variety of trachyte, the 

 ground returns a hollow sound when struck, indicating a cav- 

 ernous basis, the gases collected are a little muriatic acid and 

 much sulphuretted hydrogen from the decomposition of 

 which, as the author ingeniously reasons, the abundant sul- 

 phur of this place arises, and not from sublimation of free sul- 

 phur. The Puzzolana so much celebrated in other countries 

 as an ingredient in hydraulic mortar " is a formation of volca- 

 nic tuff, bearing many analogies to the trass of the Rhine and 

 the pumiceous conglomerates of Hungary." 



"The height of this tuff, in many places near Naples, is very 

 considerable ; the hill of the Camalduli, the loftiest eminence 

 next to Vesuvius in the whole country, is composed of it, and to 

 the west of Naples it forms a sort of wall, so lofty and abrupt, 

 that the former inhabitants of the country apparently found it 

 easier to avail themselves of the soft and friable nature of the 

 stone, and to cut through, than to make a road over it. 



" This is the origin of the celebrated Grotto of Posilippo, a 

 cavern three hundred and sixty-three toises, or two thousand one 

 hundred and seventy-eight feet in length, fifty feet in height, and 

 eighteen in breadth, which strikes every stranger with surprise 

 from the mass of rock cut through, until he reflects at the ease 

 with which a stone of such a description admits of being hollow- 

 ed out. 



" This immense mass of Puzzolana forms some considerable 

 hills round Naples, many of which, as the Monte Barbara, As- 

 troni, and others, have very regular craters, but do not appear 

 to have thrown out any currents of lava." 



