Prof. T. G. Bonney — Ligurian and Tuscan Serpentines. 363 



mineral, which is probably a variety of hornblende. In these are 

 several rather large grains of decomposed ilmenite, some grains 

 resembling a decomposed diallage, and a considerable quantity of a 

 hornblendic mineral, showing here and there distinctly the cleavage 

 of hornblende ; in other parts in rather fibrous or filmy patches, 

 many of which show a peculiar deep blue colour. On testing the 

 last for dichroism, we observe them change from this colour to 

 a pale yellowish olive, and find that with ordinary transmitted light 

 the latter tint is seen in sections cut approximately parallel to a 

 basal plane, while the blue colour appears to belong to most sections 

 cut at right angles to it. This mineral appears, from its association 

 and mode of occurrence, to be of secondary formation, as the horn- 

 blendic constituent usually is in a gabbro. There can be little doubt 

 it is glaucophane. 1 A grain or two of epidote, and various minute 

 decomposition products, are present. Though no felspar is now to 

 be distinguished, I think the general habit and structure of the rock 

 justifies us in assuming that this mineral was formerly present, and 

 we may accordingly call it a glaucophane-gabbro. 



As regards the brecciated rock mentioned above, the impossibility 

 of obtaining a thin slice makes it difficult to arrive at a conclusion. 

 The grains are not markedly angular or scoriaceous in aspect. Ilmenite, 

 glaucophane, and a pyroxenic mineral can be recognized, with pos- 

 sibly bits of serpentine and of a hornblendic or chloritic rock. There 

 is nothing about it specially suggestive of a volcanic origin ; and if it 

 be an agglomerate, I feel certain it is connected with the focus of the 

 gabbro, not of the serpentine. 



The included slaty rock gives indications of fragmental origin, but 

 has been much altered, recalling a little some of the ashy rocks of 

 the North Wales Lower Silurian. At present, however, it is chiefly 

 composed of fibrous hornblendic minerals, among which is glauco- 

 phane, and granular earthy minerals, ferrite, etc. A grain of decom- 

 posed diallage (?), associated with glaucophane, is present. 



Beyond this place we find on the shore of the Bay, near Pra, a bold 

 eminence (crowned by an old fort) rising above the sands, and now 

 isolated by a railway cutting. It is composed of a brecciated serpen- 

 tine ; the fragments, angular and varying from less than an inch 

 to a foot in diameter, are cemented by a whitish mineral, which 

 appears to be sometimes steatite, sometimes calcite or aragonite ; the 

 whole mass is in a very decomposed condition, but it appeared to me 

 certain that it had been brecciated in situ. 



To the west of this spot the shore becomes for a while flat and sandy. 

 The serpentine obviously extends inland for a considerable distance, 

 but as all that was visible appeared to be much decomposed, and 

 there were no signs of quarries, I did not quit the coast. Even 



1 An aluminous variety of hornblende ; it has been found at Zermatt (see Dana, 

 Text-Book of Mineralogy, p. 277), and at Syra (Zeitsch. deutsch. geol. Ges. Bd. 

 xxviii. heft 2, p. 248). For the above determination, and the opportunity of com- 

 paring specimens, I have to thank Mr. T. Davies, F.G.S., of the British Museum, 

 to whom, as not seldom before, I had recourse when in perplexity as to the name of 

 the mineral. 



