Panr VIII. §3.] REGIONAL METAMORPHISM. 583 
mineral solutions. The plentiful diffusion of crystalline microliths 
‘in some clay-slates and even of recognizable microscopic crystals 
(garnet, &c.), with the retention of the ordinary characters and even 
fossil contents of clastic rocks, points to a more pronounced change, 
viz. the initiation of a general crystalline rearrangement, apart from 
the mere intrusion of eruptive matter. All that is known of the 
probable origin of these minerals negatives the supposition that they 
could have been formed in the original sediment of the sea bottom 
on which the organisms entombed in the deposits lived and died. 
For their production a temperature and a chemical composition of 
the water would seem to have been required such as must have been 
inimical to the co-existence in the same water of such highly 
organized forms of life as brachiopods and trilobites. ‘Two regions 
may be cited here as affording proof of an extensive conversion of 
ordinary sedimentary strata of Paleeozoic age into crystalline schists 
—the Highlands of Scotland and the Green Mountains of New 
England. 
Hvidence from the Scottish Highlands.—In geological 
structure Scotland presents three parallel zones, which cross the island 
from south-west to north-east. The southernmost of these consists chiefly 
of greywacke, grit, and shale, with some thick lenticular seams of lime- 
stone in the south-western part of the area. These rocks have yielded 
an abundant suite of organic remains, which prove them to be of Lower 
Silurian age. They have been extensively plicated into innumerable 
anticlinal and synclinal folds, often sharp and steep, not infrequently 
reversed (p. 518). The general persistent direction of the axes of those 
folds is N.H. and §.W., and as the tops of the arches have been greatly 
denuded, the Silurian belt appears to be made up of highly-inclined and 
even vertical strata. ‘I'he central zone of the country, consisting of Old 
Red Sandstone, Carboniferous, and Permian formations, with abundant 
associated volcanic rocks, extends as a band about fifty miles broad, 
separating the Silurian uplands of the southern zone from the Highlands. 
The last-named region, occupying more than half of the whole country, 
consists mainly of crystalline schists with bosses of granite, porphyry, &c. 
These rocks stretch through four degrees of latitude, and four and a half 
of longitude, and must cover an area of not less than 16,000 square miles 
at the surface, but as they sink beneath later formations, and as they are 
prolonged into Ireland, their total area must be still more extensive. It 
was formerly believed that the crystalline schists of Scotland belonged 
to the early geological period in which such rocks were supposed to have 
been everywhere formed. Murchison, however, found the key to their 
structure, and proved them to be mainly of Lower Silurian age—the 
metamorphosed equivalents of the scarcely altered Lower Silurian strata 
in the southern zone of the kingdom. 
The oldest rock of the whole region (a, Fig. 300) is a remarkably coarse 
_ erystalline gneiss seen in Sutherland and Ross, the two north-westerly 
counties of Scotland. It will be described in the section on Archean rocks 
in Book VI. It is unconformably overlaid by nearly flat brownish-red 
(Cambrian) sandstones, conglomerates and breccias (6) which in turn 
are surmounted unconformably by inclined beds of quartzite and lime- 
