Parr VIL §i.] THE CRYSTALLINE SCHISTS. "569 
which has been preserved as a sandy shale (F), containing Calamites, 
Producti, ganoid scales, and other traces of the terrestrial and marine 
life of the time. Finally a sheet of lava, represented by the uppermost 
amygdaloid (C), overspread the area, and sealed up these records. of 
Palzozoic history.} 
Part VIII.—THE CRYSTALLINE SCHISTS AS PART OF THE ARCHI- 
TECTURE OF THE EARTH’S CRUST.—METAMORPHISM, LocAL AND 
REGIONAL. 
§ I. General Characters. 
Possessing characters which on the one hand link them with 
stratified, on the other with eruptive rocks, the crystalline schists 
present a peculiar type of structure with which are connected some 
of the most perplexing problems of geology. These rocks cover 
extensive areas of the surface of the continents, occurring usually 
wherever the oldest formations have been brought to the light. But 
they everywhere pass under younger formations, so that their visible 
superficies is probably but a very small part of their total extent. In 
the northern regions of Europe and of North America they spread 
over thousands of square miles, forming the tableland of Scandinavia, 
the Highlands of Scotland, and a great part of Eastern Canada and 
Labrador. They likewise commonly rise to the surface along the 
axes of great mountain chains in all quarters of the globe. So 
persistent are they that the belief has arisen that they everywhere 
underlie the stratified formations as a general foundation or platform. 
Some details of their structure will be given in the description of 
Archean rocks in Book VI. 
The most distinctive character of the schists is undoubtedly their 
foliation (p. 118). They have usually a more or less conspicuous 
crystalline structure, though occasionally this is associated with 
traces and even very prominent manifestations of clastic ingredients 
(pp. 123, 125). Their foliated or schistose structure varies from 
the massive type of the coarsest gneiss down to the extremely 
delicate arrangement of the finest taleose or micaceous schist. They 
occur sometimes in monotonous uniformity; one rock, such as gneiss 
or mica-schist, covering vast areas. In other places they consist of 
rapid alternations of various foliated masses—gneiss, mica-schist, 
clay-slate, actinolite-schist, and many other species and varieties. 
Lenticular seams of crystalline limestone or marble, usually with 
some of the minerals mentioned on p. 114, sometimes strongly 
graphitic, not unfrequently occur among them, especially where 
they contain bands of serpentine or other magnesian silicates. Thick 
irregular zones of magnetite, hematite, and aggregates of hornblendic, 
pyroxenic, or chrysolitic minerals likewise make their appearance. - 
1 See “Memoirs of Geol. Survey, Geology of Edinburgh,” pp. 45, 58. Trans: Roy. 
Soc. Edin. xxix. p. 483. 
