CHAP. VIII 
DALRADIAN SCHISTS 
125 
111 addition to the sills there occur also bosses of similar material, which 
ill their form and their olivions relation to the sheets recall the structure 
of volcanic necks. They consist of hornblendic rocks, like the sills, hut are 
usually tolerably massive, and show much less trace of superinduced foliation. 
Hesides the obviously eruptive masses there is another abundant group 
of rocks which, I believe, furnishes important evidence as to contemporaneous 
volcanic action during the accumulation of the Dalradian series. Through- 
out the Central and South-Western Highlands certain zones of “ green schist ” 
have long occupied the attention of the officers of the Geological Survey. 
They occur more especially on two horizons between the Loch Tay Limestone 
and a much lower series of grits and fine conglomerates, which run through 
the Trossachs and form the craggy ridges of Ben Ledi, Ben Yoirlieh and 
other mountains near tlie Highland border. In the lo\ver group of “ green 
schists,” thick hornblendic sills begin to make their appearance, increasing 
in number upwards. The upper group of “ green schists ” lies between two 
bands of garnetiferous mica-schist, above the higher of which comes the 
Loch Tay Limestone. The peculiar greenish tint and corresponding mineral 
constituents of these schists, however, are likewise found diffused through 
higher parts of the series. 
So much do the “ green scliists ” vary in structure and composition that 
no single definition of them is always ap^ilicable. At one extreme are 
dull green chlorite-schists, passing into a “ potstone,” which, like that of 
Trondhjem, can be cut into blocks for architectural purposes.^ At the 
other extreme lie grits and quartzites, with a slight admixture of the same 
greenish-coloured constituent. Between these limits almost every stage may 
be met with, the proportion of chlorite or hornblende and of granular or 
pebbly (|uartz varying continually, not only vertically, but even in the 
extension of the same bed. The quartz-pebbles are sometimes opalescent, 
and occasionally larger than peas. An average specimen from one of the 
zones of “ green schists ” is found, on closer examination, to he a thoroughly 
schistose rock, composed of a matrix of granular quartz, througli which 
acicular hornblende and biotite crystals, or actinolite and chlorite, are 
ranged along the planes of foliation. 
That these rocks are essentially of detrital origin admits of no doubt. 
They differ, however, from the other sedimentary members of the Dalradian 
series in the persistence and abundance of the magnesian silicates diffused 
through them. The idea which they suggested to my mind some years ago 
was that the green colouring-matter represents fine basic volcanic dust, 
which was showered out during the accumulation of ordinary quartz-- 
ose, argillaceous and calcareous sediments, and that, under the induence 
of the metamorphism which has so greatly affected all the rocks of the 
region, the original pyroxenes and felspars suffered the usual conversion 
into hornblendes, chlorites and micas. This view has occurred also to my 
colleagues on the Survey, and is now generally adopted by them. 
* From such a rock, wliicli ci' 03 se.s the upper part of Locli Fyiie, the Duke of Argyll’s residence 
Ht Inveraray lias lieen built. 
