362 Prof. T. G. Bonney — Ligurian and Tuscan Serpentines. 



IV. — Notes on some Ligurian and Tuscan Serpentines. 

 By Professor T. G. Bonney, M.A., F.E.S., etc. 



NOTWITHSTANDING the distinct assertion by more than one 

 geologist ' of the intrusive character of the serpentines of these 

 districts, some uncertainty seems to exist on this point and still 

 more as to the original character of the rock. Hence the result of 

 my examination of a few localities, and of my subsequent studies of 

 the rocks then obtained, may be of sufficient interest to justify 

 publication. 



I commence with the serpentine on the sea-shore to the west of 

 Genoa. Quitting that town by the road to Pegli, serpentine first 

 occurs a little west of Conegliano, where a small boss forms a head- 

 land by the sea. The rock is of a dull greenish colour, and is so 

 greatly decomposed as to be worthless for microscopic examination. 

 Still, the general aspect, form, jointing, etc., are quite those of the 

 Lizard serpentine. I have little doubt that the mass is intrusive, 

 though buildings, etc., prevented me from finding an actual junction 

 with the neighbouring sedimentary rock. This can be seen within 

 a yard of the serpentine. It is an indurated shale of schistose 

 aspect, much shattered, and traversed by calcareous veins, looking 

 in short as if it had been affected by the intrusion of an igneous 

 rock. Beyond Pegli, serpentine is again found on the sea-shore. 

 Here (just after passing some houses) is a considerable exposure of 

 serpentinous breccia, which can be studied in the cliff and in some 

 small skerries. This at first sight closely resembles an agglomerate, 

 being composed of fragments of serpentine, with some of gabbro, 

 and a few of a dark slaty rock in an ashy-looking paste, reminding 

 me by its aspect of one of the " necks " so common on the Fifeshire 

 coast. As I have long been anxious to ascertain whether serpentine 

 ever occurs as a truly eruptive rock, — i.e. as an altered lava flow, 

 tuff or agglomerate, — I examined this section with great care, but 

 was unable in the field to come to a positive conclusion. The gabbro 

 is probably the latest rock, being intrusive in the breccia, and, as I 

 think, in the serpentine also, though the latter point was not quite 

 so clear to me as the former. 



The next two headlands to the west mainly consist of gabbro. 

 This rock varies from a fine-grained variety of a dark greenish to 

 bluish colour — at a short distance hard to distinguish from the 

 serpentine — to a coarse variety composed of a white saussuritic 

 mineral and a dark green or almost black diallage or augite. The 

 latter rock much resembles some of the gabbros on the Cornish coast, 

 except that the pyroxenic mineral is less metallic and the "saussurite" 

 less abundant. It includes one or two fragments of a slaty rock 

 of rather serpentinous aspect. I have examined a slide of the 

 more compact gabbro. At a glance it is seen to have been highly 

 altered. The ground-mass consists of a rather confused mixture of 

 a clear and sometimes granular mineral, and a pale, rather fibrous 



1 D'Achiardi, vol. ii. p. 180. Stoppani, Corso di Geologia, iii. § 701. Jervis, 

 Quart. Journ. Geol. Soc. vol. xvi. p. 480. 



