228 Geological Society. 
February 11.—Prof. T. G. Bonney, D.Sc., LL.D., F.B.S., Peo 
in the Chair. 
The following communications were read :— 
1. “The Tertiary and Older Peridotites of Scotland.” By John 
W. Judd, F.R.S., Sec. GS. 
The very interesting rocks known as “ peridotites” have been 
regarded by many petrographers as peculiar to, and, indeed, cha- 
racteristic of, the older geological periods; but in the Western 
Isles of Scotland there occur a number of rocks of this class, con- 
stituting portions of intrusive masses, which the author, in a pre- 
vious paper, has shown to be the central cores of Tertiary volcanoes 
of vast dimensions. 
These Tertiary peridotites are most intimately associated with 
the gabbros and dolerites, the felspathic and non-felspathic rocks 
passing into one another by insensible gradations, and the rocks of 
either class being intersected by veins of the other. The peridotites 
exhibit the same varieties of microscopic structure as the associated 
gabbros and dolerites, these structures being described under the 
names of “ granitic,” “‘ ophitic,” and ‘“ porphyro-granulitic.” 
The felspars, which are rare in the peridotites, are intermediate in 
composition between labradorite and anorthite ; they rarely, however, 
exhibit evidence under the microscope of being built up of laminze 
belonging to different species. The study of the lamellar twinning, 
which is a common, but by no means universal, character in these 
felspar crystals, points to the conclusion that it has been induced by 
pressure or strain, like the similar structure in rock-forming calcite. 
The pyroxenes are represented by many varieties, both of the 
monoclinic forms (augites) and the rhombic forms (enstatites), the 
former being by far the most abundant. The olivines below are, 
for the most part, highly ferruginous varieties. The spinellids, 
magnetite, chromite, and picotite occur in these rocks, as do also 
titano-ferrite and its alteration-products. Among the accessory 
constituents biotite is the most abundant. 
It was shown that each of the minerals of these rocks is four 4 
undergo remarkable changes as we pass from the superficial t { 
central portions of these intrusive rock-masses. The most in % 
tant of these changes is that for which the author proposed : 
name “schillerization.” It consists in the development of micre 
scopic enclosures, in the form of plates and rods, along certain | 
planes within the crystal, giving rise to metallic reflections or a play ' 
of colour. The felspars, pyroxenes, and olivines are all found to be © 
affected in this way when they have formed the deepest parts of 
these volcanic cones. In this way common augite is seen at 
gradually increasing depths, passing into the deep brown variety 
known as pseudo-hypersthene. The last-mentioned substance 
presents a curious “mimicry” of true hypersthene and paulite, 
which is the schillerized form of a ferriferous enstatite. 
The Tertiary peridotites present many variations, not only in 
