410 Prof. T. G. Bonney — Remarks on Serpentine. 



explanation was admissible, in four the serpentine was probably 

 intrusive, in one certainly so. 



Ball/. — (Near Genoa) the serpentine is probably intrusive, (near 

 Moate Ferrato) probably intrusive, (near Figline) intrusive. As Dr. 

 Hunt has visited this last massif, I will repeat my reasons for 

 maintaining the intrusive character in this case. A rouoh sketch 

 of what I saw will be found in this Magazine on p. 369 of Vol. 

 VI. (Dec. II.). A little crag of bedded argillite overlies a mass of 

 characteristic (though rather rotten) serpentine. The base of the 

 crag is masked by a bank of debris, so that the actual junction could 

 not be seen without excavation, but the sedimentary rock has been 

 indurated and been cracked ; it has, in short, that ' baked ' look 

 which is so familiar to any one who has worked much at junctions 

 of igneous and argillaceous rocks, such, for instance, as we obtain 

 among the Carboniferous rocks of the Central Valley in Scotland. 

 These beds, however, within three or four feet vertically above the 

 serpentine, contain organisms, among them probably Polycystina ; 

 though these, as we might expect from the proximity of an intrusive 

 rock, are not very perfectly preserved. About six paces from this 

 craglet, an isolated slab of dull red sedimentary rock still adhered to 

 the serpentine. This I have examined under the microscope ; it is 

 an impure, somewhat crystalline limestone ; its position and aspect, 

 macroscopic and microscopic, seemed and still seem to me inexplic- 

 able on any hypothesis except that of the intrusion of the serpentine. 



Dr. Hunt speaks of the existence of an arenaria opMolittca as 

 vouched for in other localities by Italian geologists ; but knowing 

 how vaguely this term is used, I must ask for more information 

 before I can take it into consideration. He speaks also of having 

 seen a breccia of serpentine at the base of the sedimentary rock. 

 This certainly does not occur at the above locality, and from what 

 I have elsewhere seen I would imitate his method of geological 

 criticism so far as to ask whether it might not be explained other- 

 wise than by sedimentation. Serpentine is a rock peculiarly liable 

 to brecciation, and I have seen more than one curious breccia of it 

 and the adjacent rock, which I had no doubt was due to subsequent 

 crushing,' At any rate, whatever be the case elsewhere, here at 

 Figline, I cannot explain what I saw on any other theory than 

 that of an intrusion of the serpentine into the sedimentary group. 



Next, as to the genesis of serpentine. Here I will simply recapitulate 

 the evidence which I have set forth at great detail in various jJfipers. 



Olivine-gabbros and troktolites, which are frequently intrusive, 

 illustrate perfectly the passage from olivine to serpentine in the case 

 of that one mineral. Peridotites (like the Iherzolite of the Ariege) 

 show the first stages of the same change. 



Serpentines like those of Coverack (Lizard), Levanto (Eiviera), Sta, 

 Caterina (Elba), Baste (Hartz), etc., often contain half the olivine still 

 unchanged, and starting from these we can trace all stages of the 

 alteration till every particle of the olivine has disappeared, I may add 



1 Serpentine under pressure sometimes assumes a schistose aspect. This has 

 added to the difficulties of distinguishino" the rock from simulative forms. 



