THE STRUCTURE AND CLASSIFICATION OF ROCKS. 



629 



at the surface, as at e, it is said, in the language of miners, to crop out. Sometimes 

 several of the diversities in the structure of 'strata noticed, appear associated together, 

 as in fig- 25., an appearance very common in the slate rocks on the north coast of Devon. 

 It is obvious that the disturbances which have originated these constructions, have ren- 

 dered an essential service to mankind, have largely opened the surface of the earth to 

 their knowledge, and brought the mineral riches of different strata, which lie low in | 

 the series, within reach ; for had they remained in an undisturbed horizontal position, 

 piled one upon another, man could never have penetrated to those which are most 

 important to the arts of life, and would for ever have been ignorant of their existence. 



The operation of an elevating cause, to which various stratified formations have been 

 subject, has frequently interrupted their continuity, produced cracks and fissures, filled 

 with fragments of rocks from above, or with igneous injections from below, extending in 

 breadth from a few inches to twenty or thirty feet, and even yards. These are termed 

 faults, slips, notches, or dykes. The shock which has produced this dislocation of a 

 stratum, besides interrupting its continuity, has commonly also caused a change of level 

 in the divided parts. Fig. 26. is an example of a fault, where 

 beds that were once united appear broken, the separated por- 

 tions being displaced, one rising above the other, which is, in 

 mining language, termed the up-throw or down-throw of a bed. 

 26. The occurrence of faults, which are very common in the coal 



formation, is a source of great inconvenience to the miners, not only interrupting their 

 labours, but involving them in perplexity, as to whether the continuation of 'the mineral 

 is to be sought upon the same level, or whether it lies above or below it. Several faults 

 occurring in a stratum, where the divided portions are variously altered in their level, 

 often engender the hope of great mineral riches in a district, which is deceptive. Thus 



Jig. 27. shows dislocation, giving 

 to the same series of beds the ap- 

 pearance of many. It might be 

 imagined by a sanguine specu- 

 lator, observing these beds crop 

 out at the surface at a, b, c, and d, that here wcrs four distinct seams of coal, whereas 

 he would find, upon working them, that his calculation was false as to the wealth of his 

 land, for there is but a single seam, which has been dislocated and variously disturbed. 



It will be at once perceived, that the upheaving cause which has given rise to faults in 

 strata, has operated to produce the fractured appearance of the surface of the earth, 

 which is the aspect of many valleys, subsequently modified and rendered beautiful and 

 arable by atmospheric and aqueous agencies. In the early stages of geological inquiry, 

 the formation of all valleys was referred to the erosion of river currents : but this is 

 plainly an inadmissible theory ; for an immense number of valleys are found at high eleva- 

 tions, where there are no streams, and never could have been any, of sufficient power to 

 scoop them out. Some of those in low countries may have been produced by this cause; but 

 even the valleys of denudation, as they are termed, appear to be original hollows in the 

 superficies, which the rains and rivers have contributed to modify and enlarge. The 

 longitudinal hollow spaces between two ranges of mountains, and the transverse breaks 

 which interrupt the continuity of the same chain of hills, often constituting mere gorges 

 and ravines, exhibit in most cases such similarity of strata on the opposite sides, and 

 appear so capable of interlocking, if brought together, as to leave no doubt of their 

 formation resulting from the breakage of the general mass during its upheaval. The 

 sides of such valleys, ravines, or gorges are commonly excessively steep and rugged; 



S 8 3 



Fig. 27. 



