OLDEE PEKIDOTITES OV SCOTLAND. 



397 



unaltered. ]Vrorcover, the change is, in many cases, seen to be set 

 up from the surfaces or fissures of the minerals to which percolating 

 atmospheric waters can most easil}^ find access. It will be instruc- 

 tive to study the changes which the several minerals are found 

 undergoing, under these circumstances, in the Palaeozoic peridotites. 



Olivine is the mineral in the peridotites which undergoes 

 change most easily, and in almost every case it is found converted 

 into serpentine ; indeed it is quite rare to find examples of the un- 

 altered olivine in these Palaeozoic rocks. The occurrence of unmis- 

 takable pseudomorphs of serpentine after olivine, and the occurrence 

 of particles of unaltered olivine in the midst of the serpentine 

 masses, afi'ord abundant evidence, however, of the fact that the 

 serpentine is for the most part altered olivine. 



When the olivine has underoone the changes described in the 

 first part of this paper, and as a consequence contains stellate, 

 tabular, and irregular enclosures of magnetite and other oxides, 

 these are sometimes seen to persist after the hydration of the enclos- 

 ing mineral. But the conversion of the olivine into serpentine, as 

 is well known, is usually accompanied by a separation of magnetite, 

 the silica combined with the iron of the original mineral being pro- 

 bably to some extent carried away in solution. In many cases it 

 appears to be impossible to separate the mixed oxides formed during 

 the Schillerization of the olivine from those liberated during its 

 s erpentini z ation . 



The lihomhic Pyroxenes {Enstatites) undergo change much less 

 rapidly than do the olivines. This is shown by the fact that in 

 rocks which have originally consisted of olivine and enstatite, the 

 former mineral is often entirely changed to serpentine, while the 

 latter remains comparatively unaltered. 



The change which the enstatites undergo seems to vary in different 

 cases. Mr. G. H. AVilliams has described an interesting example 

 of the direct conversion of a ferriferous enstatite (hypersthene) 

 into a brown hornblende *. But of this kind of change I have not 

 found any examples among the numerous enstatites of the Scottish 

 rocks. On the contrary, the change in the mineral appears usually 

 not to be a simply molecular alteration, but to be the result of 

 hydration. Thus in the enstatite-basalt of a dyke at Carroch in 

 Forfarshire, the fine crystals of highly ferriferous enstatite are found 

 passing along their edges and fissures into a green substance un di- 

 stinguishable from that found in similar situations in olivine crystals, 

 and in some cases the whole crystal is converted into this substance. 

 Again, in the serpentine dyke of the same district, the central and 

 least altered portion consists of serpentine crowded with enstatite 

 crystals, but in the more altered portions of the same mass, the 

 enstatite is seen to pass into serpentine f. 



* Amer. Journ. Sci. 3rd ser. vol. xxviii. (1884) p. 262. 



t According to my own experience, the rhombic pyroxenes are generally 

 converted into a serpentinous material, while the augites pass into a uralitic 

 substance or directly into hornblende ; and I would venture to suggest that 

 the crystals which Mr. Williams describes as changing into hornblende, in the 

 passage referred to, may be pseud o-hypersthene, and not true hypersthene. 



