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480 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IY. 
youngest. The cause of this structure is not well understood. 
Among the sands and clays of the glacial deposits local examples 
of contortion occur, which may be accounted for, in some cases, by 
the intercalation and subsequent melting of sheets of frozen mud ; 
in others by the stranding of heavy masses of drift ice upon still 
unconsolidated sand and mud. It is possible that some of the 
extraordinary labyrinthine and complex contortions of schistose rocks 
may be due to the subsequent crumpling of strata already full of 
_ this diagonal contorted lamination. 
Irregularities of Bedding due to Inequalities of 
Deposition or,of Hrosion.—A sharp ridge of sand or gravel 
may be laid down under water by current-action of some strength. 
Should the motion of the water diminish, finer sediment may be 
brought to the place and be deposited around and above the ridge. 
In such a case the stratification of the later accumulation will end 
SSS 
Yaros 

100 200 Goo 4000 
Fic. 191—PLan or CHANNELS IN CoAL, Forest or DEAN (AFTER BUDDLE), 
off abruptly against the flanks of the older ridge, which will appear 
to rise up through the overlying bed. Appearances of this kind are 
not uncommon in coal-fields, where they are known to the miners as 
“rolls,” “swells,” or “ horses’ backs.” A structure exactly the reverse 
of the preceding where a stratum has been scooped out before the 
deposition of the layers which cover it, has also often been observed 
in mining for coal, when it is termed a “ want.” Channels have been 
cut out of a coal-seam, or rather out of the bed of vegetation which 
ultimately became coal, and these winding and branching channels 
have been filled up with sandy or muddy sediment. The accom- 
panying plan (Iig. 191) represents a portion of a remarkable series of 
such channels traversing the Coleford High Delf coal-seam in the 
Forest of Dean. The chief one, locally known as the “ Horse” (a6), 
has been traced for about two miles, and varies in width from 170 to 
340 yards. It is joined by smaller tributaries (¢¢), which run for some 
way approximately parallel to it. ‘The coal has either been prevented 
