570 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
Another characteristic of the schists is their usual intense 
erumpling and plication. The thin folia of their different component 
minerals are intricately and minutely puckered (Fig. 19). Thicker . 
bands may be traced in violent plication along the face of exposed 
crags. So intense indeed have been the internal movements of these 
masses that the geologist experiences great and often insurmountable 
difficulties in trying to make out their order of succession and their. 
thickness. Such evidence of disturbance, though usually strongly 
marked, is not everywhere equally so. Some areas have been more 
intensely crumpled and plicated, and where this is the case the 
rocks usually present their most conspicuously crystalline structure. 
A further eminently characteristic feature of the schists is their 
common association with bosses and veins or bed-like sheets of 
granite, syenite, quartz-porphyry, or other massive rocks. In some 
regions indeed so abundant are the granitic masses and so coarsely 
crystalline or granitoid the schists, that it becomes hardly possible 
to draw satisfactory boundary lines between the two kinds of rock. 
Apart from disputed theories as to the mode in which the crystal- 
line schists were formed, there seems no good reason to doubt that 
originally these rocks were laid down in sheets or beds, and that their 
present puckered and plicated condition has been the result of 
terrestrial movements similar to those by which the crumpling and 
plication of ordinary sedimentary rocks in mountain regions have 
been produced. The alternations of different bands of quartzose, 
aluminous, or magnesian composition, with the occasional intercala- 
tion of lenticular zones of white marble, at once recall the manner 
in which deposits of sandstone and shale, associated with each other 
in the older geological formations, are here and there interrupted 
by courses of limestone. This first postulate, therefore, is generally 
granted, that the crystalline schists were deposited on the sea-floor. 
But the next step in the induction has given rise to great 
differences of opinion. Some geologists maintain that the crystalline 
schists are original chemical deposits of the primeval ocean. Others 
insist that these rocks were at first mere mechanical, possibly to 
some extent chemical, sediments, and that their present crystalline 
and foliated characters have been superinduced upon them ; in other 
words that they are metamorphic rocks. One of the chief causes of 
the difficulty of the problem lies in the fact that the crystalline 
schists are in the majority of cases separated from all other geo- 
logical formations by an abrupt hiatus. Instead of passing into 
these formations they are commonly covered unconformably by 
them, and have usually been enormously denuded before the deposi- 
tion of the oldest overlying rocks. Hence, not only is there a want 
of continuity between the schists and younger formations, but the 
contrast between them in regard to lithological characters and 
geotectonic structure is so exceedingly striking as naturally. to 
suggest the idea that the schists must belong to a period long 
anterior to that of the earliest sedimentary formations of the ordinary 
