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592 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV, _ 
How simply this irregularity may be accounted for, will be readily 
perceived by merely copying the line of such an uneven fissure on 
tracing paper and shifting the tracing along the line of the original. 
If, for example, the fissure be assumed to have the form shown at ab, 
in the first line (Fig. 301), a slight shifting of one side to the right, as 

Fic. 301.— WIDENING OF A FIssURE BY RELATIVE SHIFTING OF ITS SIDE (B.). - 
at a’ b',in the second line will allow the two opposite walls to touch at 
only the points o 0, while open spaces will be left ateed. A movement 
to the same extent in the reverse direction would give rise to a more 
continuously open fissure as in the third line. That shiftings of 
this nature have occurred to an enormous extent in the fissures filled 
with mineral veins is shown by the abundant slickensides (p. 504). 
The polished and striated walls have been coated with mineral 
matter, which has subsequently been similarly polished and grooved 
by a renewal of the slipping. 
Structure and Contents.—A mineral vein may be either 
simple, that is, consisting entirely of one mineral, or compound, 

Fia. 302.—Sncrion of A Fissure NEARLY FILLED WITH ONE MINERAL (ce ¢), 
BUT WITH A PORTION OF THE FISSURE (@ b) STILL OPEN (B.). 
consisting of several, and may or may not be metalliferous. The 
minerals are usually crystalline, but layers or irregular patches of 
soft decomposed earth, clay, &c., frequently accompany them. The 
non-metalliferous minerals are known as veinstones, the more 
crystalline being often also popularly classed as spars. The metal- 
bearing minerals are known as ores. ‘The commonest veinstones 
are quartz, calcite, barytes, and fluorite. The ores are sometimes 
native metals, especially in the case of copper and gold; but for 
the most, part are oxides, silicates, carbonates, sulphides, chlorides, 
or other combinations. Of the manner in which the contents of a 
mineral vein are disposed, the following are the chief varieties, 
