376 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 27, 



here exhibited, it is certain that this eruption of " gabbro " took place 

 after the consohdation of the alberese and macigno formations, i. e. 

 after the younger chalk and older eocene. It is also further evident 

 that another movement of elevation occurred after the miocene pe- 

 riod ; for not only is the limestone associated with white marls to a 

 great extent loaded with alabaster, which some persons might infer 

 was altered limestone, but the whole of this mass has been considered 

 to be miocene, simply because it dips away from the alberese and 

 gabbro in such inclined strata, and is thus placed in striking contrast 

 with the subapennine or pliocene marls of the valley which surround 

 a boss of " gabbro rosso" in perfectly horizontal and unbroken layers. 

 The altered alberese at Civita Castellina has here and there serpen- 

 tinous soft bands, and bears a metamorphic aspect, with a slickenside 

 surface, accompanied by cracks and numerous veins of arragonite, all 

 of which specially abound near the junction of the alberese with the 

 "gabbro." Copper veins, however, either traverse the alberese or 

 run down its junction with the gabbro ; and are therefore of date 

 posterior to the eruption of the latter. It is indeed the opinion of 

 Pilla, that the copper veins have resulted from the same igneous action 

 which evolved the "'gabbro rosso," and are contemporaneous with 

 that rock, whilst other authors contend that they are posterior to it. 

 In traversing on foot the wild ridges which separate Civita Castellina 

 from Monte Catini, where the richest copper ores abound, I witnessed 

 repetitions of the chief phsenomena above alluded to, in which, be- 

 sides "gabbro rosso" and felspathic trap (the epidosite of Pilla), 

 there were other rocks of this class both of greenish and purple co- 

 lours, which I should class as greenstone and serpentine. All these 

 amorphous masses, however diversified in aspect and structure, seemed 

 to me to form parts of the same eruptive matter which has pene- 

 trated the macigno and alberese in lines from N.N.W. to S.S.E. 



At Monte Catini, where Mr. Hamilton seems to have most studied 

 it, the gabbro rosso appears in a bold promontory fronting the valley 

 of the Cecina on the south, and Volterra on the east. The chief 

 mass is here the same amorphous spheroidal variolite as in other 

 places. Partially, indeed, it assumes still more a serpentinous ap- 

 pearance ; the dull red globular lumps and spheroids being often 

 enveloped in greenish coatings. It is not my province to allude to 

 the splendid veinstones of copper*, occasionally quartzose, which 

 ramify along its edges or through this " gabbro ;" I content myself 

 with saying, in reference to the point at issue, that in numerous 

 galleries and cuttings the clearest proofs are exhibited of the homo- 

 geneity of structure of the amorphous gabbro, and of the total ab- 

 sence of anything in it like original aqueous deposit. In this respect 

 it bears no resemblance to any other metamorphosed stratum which 

 ever fell under my notice. The variolitic arrangement of the sphe- 

 roids is very striking. On exfoliation they exhibit the pustules be- 

 fore alluded to on the external surface only of each concentric fold, 



* As a wayfaring geologist, I wais most hospitably received at his villa by Mr, 

 Sloane, the intelligent proprietor of the copper mines of Monte Catini. The ore 

 is very peculiarly diffused and merits a special study. 



