1841.] 471 



" and particularly of its strata of schistose clay and limestone ; an 

 " alteration caused by the ophiolitic rocks, and consisting not only 

 " in the induration and liver-red colouring of the strata, and in 

 " their contortions, but often in a perfect fusion or amalgamation of 

 "the neptunian with the plutonic rocks ; by these means, many 

 "rocks of an ambiguous and varied appearance were produced, in 

 " which, at one time, the elements of the neptunian rocks pre- 

 " dominate, and at others, those of the plutonic, so that in former 

 ■ " times they were said to belong to the transition formations. We 

 " have adopted for this class of products the generic name of 

 " gahbro rosso, being that which is usually given to it in Tuscany." 

 The principal points where this rock is exposed are the copper 

 mines of Monte Catini, and the mountain range extending thence 

 to Castellina, besides a few spots on the south side of the Cecina, 

 such as Libbiano, Monte Castelli, and, I believe, Eocca Sillana. The 

 works at La Cava, the spot where the mining operations of Monte 

 Catini are carried on, are all built on it, and very interesting sections 

 are developed round the western extremity of this chain of hills. 

 This rock has all the characters of being of igneous origin ; but 

 from careful examination there can be no doubt that it is an 

 altered rock. The principal condition of its appearance is that of 

 large irregular spherical masses, consisting of a darkish red argillo- 

 siliceous substance, at first very hard, but becoming brittle and 

 friable after a short exposure to the air. It is traversed by nu- 

 merous small veins and filaments of calcareous spar, which faci- 

 litates its breaking in various directions. In some of these veins 

 has been discovered a peculiar mineral which has received the 

 name of Caporcianite, from the Chapel of La Madonna di Capor- 

 ciano in the immediate neighbourhood. The centre of the veins of 

 calc spar is sometimes more opaque and of a slightly redder hue ; 

 this has been analysed and found to contain a small proportion of 

 manganese. It is said to be peculiar to these mines, and thence it 

 has derived its name, these mines being under the peculiar care of 

 La Madonna di Caporciano. The outer crust of these spherical 

 masses shows after short exposure to the air a small mammillary ap- 

 pearance, which becomes larger as the exposure continues. It is 

 evidently the result of decomposition, acting upon the surface of a 

 body hardened under circumstances which produced a tendency 

 to spherical or concretionary form, a tendency proved not only by 

 this peculiarity of the surface, but by the fact of the whole of the 

 rock consisting of these spherical masses ; for not only does this 

 condition appear in sections and surfaces where the rock has been 

 long exposed, but it also occurs in fresh sections opened in the 

 very heart of the mountain, many hundred fathoms below the sur- 

 face, where the nodules or spheroidal masses, called nuoccioli by 

 the workmen, and averaging 2 or 3 feet in diameter, are dis- 

 tinctly visible in the sides of the recently opened galleries. The 

 intervening substance is generally very soft, and consists of gi'een, 

 red, and white earths, the red being generally in contact with the 

 round masses, and the green and white more in the interior. 



VOL. IV. PART III. N N 



