398 



PKOF. J. W. JTJDD ON THE TERTIAKT AND 



"When the enstatite has been previously Sehillerized, the charac- 

 teristic enclosures often persist after the serpentinization of the 

 material in which they are enclosed. We may thus sometimes 

 infer the former existence of a particular mineral in a rock by the 

 presence of the characteristic enclosures of the mineral in the midst 

 of its alteration-products. 



The Monodinic Pyroxenes (Augites) usually undergo only the 

 molecular change by which they pass into hornblendes. Thus 

 crystals of the dark brown ferriferous augite of the Shiant-Isles 

 rock are seen passing into dark-brown hornblende with the charac- 

 teristic pleochroism and cleavage of that mineral. It is worthy of 

 notice, as pointed out by Williams *, that in these cases of the con- 

 version of a pyroxene directly iuto hornblende the principal planes 

 of the two minerals are parallel, and even the planes of twinning of 

 the original may persist as such in the altered form f. 



When augite has been submitted to Schillerization before altera- 

 tion the results are of a very different kind, as is so well seen in the 

 saussurite-gabbros or wurlitzites. By the collection of the iron of 

 the mineral into the enclosures a substance is left which has the 

 composition and optical properties of diopside, and this is altered 

 into the green varieties of hornblende known as smaragdite and 

 actinolite, while the separated iron-oxides crystallize by them- 

 selves. 



On the alterations by weathering of the felspars and other acces- 

 sory minerals of the peridotites it will not be necessary to dwell in 

 detail, as they are not essential minerals of the peridotites. 



§ 2. Yabieties of the Paljeozoic Peridotites. 



Of all the varieties of the peridotite which we have described as 

 occurring among the intrusive rocks of Tertiary age, representatives 

 are found among the more or less altered Palaeozoic igneous masses. 



Eocks like the dunites, which are composed almost entirely of 

 olivine, are by hydration converted into serpentine, and some of the 

 very pure serpentine masses of Scotland were in all probabihty 

 originally dunites. 



But since enstatite, as we have already seen, is, like olivine, also 

 converted into serpentine, though somewhat more slowly, masses of 

 pure serpentine may be formed by the hydration of olivine-enstatite 

 rocks like Iherzolite. 



Admirable examples of such altered olivine-enstatite rocks have 

 been described by Professor Bonney as occurring not only at the 

 Lizard in Cornwall but at Colmonell in Ayrshire J. My own exami- 

 nation of slices taken from this serpentine leads me to conclusions 



* Amer. Journ. Sci. ser. 3, vol. xxviii. p. 264. 



t In some cases this conversion of augite into hornblende takes place directly, 

 the dichroic hornblende appearing, as it were, eating into the non-dichroic 

 augite. But in other cases the whole augite crystal appears to be converted 

 into uralitic and j&brous hornblende, and this may change subsequently into 

 common hornblende. 



J Quart. Journ. Geol. Soc. vol. xxxiv. (1878) p. 769. 



