Murchison s Silurian System. 



237 



than remove the detached masses, as these fissures are un- 

 attended with any displacement of strata. They are ascribed 

 to one of the last changes to which the strata affected by 

 them were exposed after their deposit. Crystalline forces have 

 been supposed by Professor Sedgwicke to have had some 

 effect in producing such fissures as these, as well as the 

 slaty cleavage of rocks. Mr. Murchison, however, thinks 

 that such joints in strata have been occasioned by heat, par- 

 ticularly as they seem to be more frequent in the vicinity of 

 fissures of eruption, and particularly in those strata that 

 have been altered by the effects of heat. 



Landslips of the Silurian Rocks. — These are referred by 

 Mr. Murchison to the jointed condition of the strata, and 

 their inclined position on the surface of steep ridges, toge- 

 ther with the softening or decay of subjacent beds. 



Wells in Silurian Rocks. — The jointed structure of the 

 strata already noticed, renders them permeable to water, 

 while, faults, dykes and dislocations act as dams to it, and 

 thus it is obliged to find some other outlet, thus also lines 

 of faults are often traceable by the outburst of springs ; 

 singular springs of this kind are referred to by Mr. Mur- 

 chison, of which every mountainous tract may afford instan- 

 ces. Mineral springs depend on similar causes, the pecu- 

 liar properties of their waters being derived from the strata 

 through which they permeate. 



Mining Ground of Silurian Rocks.-— This is situated in 

 the altered rocks, where they come within the influence of 

 trap. Several veins of galena, common and steel grained, 

 are worked ; also carbonate of lead, both crystallized and 

 stalactitic. The veins are chiefly parallel to the strike of the 

 strata ; but some are in the form of the letter N. Nearly all 

 the veins diverge in separate strings ; many of the best and 

 : richest ores being found in the points of intersection. The 

 ordinary ores yield from six to seven ounces of silver per 

 ton. Some of the veins were first worked by the Romans, 



