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



They fully confirm the opinions expressed above, and from the 

 appearance I should conjecture the brecciation took place when the 

 rock had become a serpentine. In one there is part of a vein (?) 

 containing a pale feebly dichroic variety of hornblende and an 

 associated mineral more like an altered enstatite ; probably they are 

 of secondary origin. With the calcite a little dolomite, I believe, is 

 intercrystallized. 



We have then here a mass of ordinary serpentine which has been 

 crushed into a breccia in situ and consolidated again by infiltration of 

 calcite. Any one who has examined serpentine much in the field 

 will remember that its sharp irregular jointing and brittle nature 

 would cause it to crush more readily perhaps than most other rocks. 

 The whole of the hilly district bordering the Riveria di Levante has 

 been greatly disturbed, and its rocks are often much contorted. 

 During one of these disturbances no doubt the crushing took place. 

 At that time limestones, which still predominate among the sedi- 

 mentary rocks around this serpentine massif, doubtless extended 

 above it, and the water which percolated downwards from them 

 while they were undergoing denudation deposited the CaC0 3 with 

 which it was charged in the fissures of the subjacent rock. This has 

 now been exposed to view, all trace of the once overlying rock 

 having disappeared. 1 



Besides the above breccia, I have examined microscopically the two 

 varieties of serpentine, described above, from Levanto. The slide 

 from the more granular rock (with crossing Nicols) is seen to consist 

 chiefly of very characteristic olivine grains, separated by threads (of 

 variable thickness) of serpentine, about equal quantities of the two 

 minerals being present. There are the usual clots of opacite. In short, 

 the appearance of the ground-mass of the slide is so similar to what 

 I have already described in a Cornish serpentine, 2 that repetition is 

 needless. Enstatite and augite are both present, as I have proved 

 by optical tests, etc., and perhaps also a little diallage. I think 

 the first mineral rather predominates, but there are difficulties in 

 determining the crystalline system of some of the grains. Endo- 

 morphs of opacite are rather commoner than is usual in the enstatite. 

 There is a little picotite. The other and compact variety exhibits a 

 more complete conversion into serpentine. No olivine remains to show 

 chromatic polarization, though here and there a grain is still doubly 

 refracting. In this also there is (as might be expected) more 

 opacite. It often forms continuous strings, is more or less present 

 in the grains (formerly olivine), being either disseminated through- 

 out them, or in bands towards the exterior. Diallage and enstatite 

 are both present, the latter being surrounded by a border of a serpen - 

 tinous mineral, into which the planes of principal cleavage are con- 

 tinned, and are often picked out by thin lines of opacite. The cleavages 



1 I should perhaps state that there is nothing to favour the idea of this crushing 

 having been sudden, or associated with any exceptional amount of heat. It may have 

 been the result of long-eontmued pressure and yielding now here now there — 

 possibly, indeed, the process of crushing and cementation may have been repeated 

 more than once. 



2 Quart. Journ. Geol. Soc. vol. x. sec. xxxiii. p. 916. 



