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120 GEOGNOSY. = ‘ -[Boox IT. 
a tough coherent whole, the word rock is usually substituted for 
schist, as in “hornblende-rock,” “actinolite-rock,” and so on. The 

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student must bear in mind that while the possession of a foliated — 
structure is the distinctive character of the crystalline schists, it Is 
not always present in every individual bed or mass associated with 
these rocks. Yet the non-schistose portions are so obviously integral 
parts of the schistose series that they cannot without great vi0- 
lation of natural affinities be separated from them. Hence in the 
following enumeration they are included as common accompaniments 
of the schists. For the same reason quartz-rock is placed in this 
subdivision, though it only occasionally shows a schistose structure. 
The origin of the crystalline schists has been the subject of long 
discussion among geologists. Werner held that, like other rocks of 
high antiquity, they were chemical precipitates from a universal 
ocean. Hutton and his followers maintained that they were 
mechanical aqueous sediments altered by subterranean heat. These 
two doctrines in various medifications are still maintained by opposite 

Tic. 19.—Conrtortrep MICACEOUS-SCHIST, AS SEEN UNDER THE MICROSCOPE 
WITH A MAGNIFYING POWER OF 50 DIAMETERS. 
schools, Some schists are undoubtedly altered sedimentary rocks, 
and may properly be termed “metamorphic.” Whether this 
has also been the origin of certain ancient gneisses and schists under- —_ 
lying the oldest fossiliferous formations is less easily determined. 
(See Book IV., Sect. viii). 
Tale-schist.—A schistose aggregate of scaly tale, often with 
quartz, felspar, and other minerals; having an unctuous feel, and 
white or greenish colour. Occurs in beds associated with mica- 
schist and clay-slate, and frequently contains magnetite, chlorite, 
mica, kyanite, and other minerals, including carbonates. A massive 
variety composed of a finely felted aggregate of scales of tale with 
chlorite and serpentine is called potstone (Topfstein). Many rocks 
have been classed as tale-schist, which contain no tale but a hydrous 
mica, ‘These are called by Dana hydro-mica-schists,  Tale-schist 
