244 



JOINTS OF THE SILURIAN ROCKS. 



the joints of the same district, though he has not separated them from veins, while Mr. 

 De la Beche and others have contributed to increase our knowledge of this difficult 

 branch of geological inquiry. 



Professor Sedgwick, in an able original memoir, has recently described those infinite 

 separating planes which constitute slaty cleavage, and has clearly pointed out the di- 

 stinction between them and the divisions of which we are now treating. Professor Phillips 

 has, however, more than any other geologist directed attention to joints, and has done 

 great service in methodizing the inquiry 1 . Seeing the surprising regularity of the di- 

 rection of the master joints in the north of England, where the prevalent direction of 

 the strata is from north to south, he suggested that the east and west direction of the 

 former might have been communicated by some general polarizing or electro-magnetic 

 influence. 



Once roused to a sense of the importance attached to jointed structure, the geologist 

 will find a new source of interest in every face of rock • he must now, indeed, perceive 

 that it is not enough simply to observe the strike and dip of the strata, he must 

 also note the direction and measure the angles of their joints. 



Referring to the table in the appendix for detailed observations, I will here merely 

 state that the Silurian rocks contain, for the most part, two sets of prevailing joints, 

 ranging in lines nearly diagonal to the strike, or axis of elevation of the strata. These 

 diagonal divisions are usually symmetrical, and preserving precise relations, so in- 

 tersect each other, as to present a number of salient and re-entering angles in the 

 escarpment of every freshly opened quarry, as represented in the above vignette. They 

 are generally vertical (or nearly so) to the surfaces of the beds ; by which arrangement 

 the rock is split into a number of lozenge-shaped masses, which are more or less well 

 defined, according to the clearness and precision of the faces of the joints. 



In some chains, indeed, but very seldom in this region, there are long fissures either 

 coincident with the line of elevation, or nearly at right angles, i. e. in the same direc- 

 tion as the dip of the strata. These doubtless may very frequently have resulted from 

 elevation of the strata "en masse," and they must not be confounded with the sym- 

 metrical joints of which we are now treating 2 . 



I therefore confine my remarks to the dominant joints, which in the ridges under 

 consideration have fixed relations to each other, though those directions change with 

 every variation of the strike of the strata. Allusion has been previously made to 

 these joints as forming the backs of the quarries in the manner represented in the 

 above vignette. They are the natural fissures by which the quarrymen are more 



1 See Phillips's Geology of Yorkshire, vol. ii. p. 90., and the same author's Guide and Treatise in Geology. 

 In these works the subject of joints is ably treated. 



2 These fissures come more properly within the class of phenomena to which Mr. Hopkins has recently so 

 well directed attention, and are more naturally connected with the preceding observations upon dislocations, 

 entirely dependent upon lines of elevation. 



