Part VIII. § 38.} REGIONAL METAMORPHISM. 589 
mountains, becoming more highly foliated and crystalline as they recede 
from the lowlands. 
Numerous bosses of granite and porphyries occur among the 
crystalline schists. But the metamorphism is not specially connected 
with their protrusion, though usually in their vicinity the schists attain 
a more largely crystalline condition. Here and there, indeed, a gradation | 
can be traced through gneiss into granite. This is more particularly 
observable in districts where veins, whether of intrusion or of segregation, 
are abundant. Remarkable examples may be observed in Hastern 
Sutherland (Lairg), and on the coast-line south of Aberdeen, where the 
gneiss loses its schistose structure, and passes into granite, which lies in 
beds intercalated in the gneiss, and in which may be seen scattered 
patches of gneiss still retaining foliation. On the other hand, some of 
the masses of granite assume here and therea perfectly gneissose structure, 
as at the large granite quarries near Aberdeen, where this structure may 
be specially observed in connection with segregation veins (Fig. 284). 
In the Scottish Highlands, therefore, it can be proved that rocks 
containing Lower Silurian fossils are overlaid by thousands of feet 
of crystalline schists, quartzites, and limestones. ‘That these overlying 
masses are not original chemical precipitates may be concluded on the 
following grounds. ist, They demonstrably overlie fossiliferous Lower 
Silurian rocks. Strata of corresponding geological age occur to a depth 
of many thousand feet in the South of Scotland, within sight of the 
crystalline rocks of the Highlands. It cannot be supposed that on the 
same sea-floor, and within the same limited area, mechanical sediments 
alone accumulated in one tract, while only a few miles distant chemical 
precipitates—eneisses, garnetiferous schists, &c.,—were laid down, in each 
case to a depth of thousands of feet. 2nd, The crystalline schists of the 
Highlands in their less altered parts present the closest resemblance to 
the ordinary greywackes, grits, and shales of the Lower Silurian series 
of the South of Scotland. Moreover, the altered rocks round the granite 
bosses in this latter area cannot be distinguished from similar rocks in the 
regional metamorphic area of the Highlands. 3rd, Throughout all parts 
of the Highland region traces of an original fragmental or clastic origin 
can be detected among the schistose rocks. Zones of fine grit full of 
well-rounded fragments of quartz, felspar, or other ingredient abound 
among the schists. Bands of coarse conglomerate likewise occur on 
different horizons, the pebbles (granite, gneiss, &c.) being enveloped in a 
schistose matrix. Microscopic investigation likewise reveals, even among 
the crystalline mica-schists, traces of the original water-worn granules of 
quartz in the sandy mud out of which the rocks have been formed. 
The conclusion is thus reached that in the Highlands of Scotland there is 
a mass of rocks originally composed mainly of ordinary mechanical 
sediments which have assumed in various degrees a crystalline condition 
over aregion which, including the north of Ireland, must cover more than 
20,000 square miles. 
Green Mountains of New England.—In this region a similar 
series of changes has been effected. The Lower Silurian strata, which 
to the north in Vermont are comparatively little changed, become increas- 
ingly altered as they are traced southwards into New York Island. 
They are thrown into sharp folds, and even inverted, the direction of 
plication being generally N.N.E. and $.8.W. This disturbance has been 
