362 



CHIEF DISTINCTIONS OF THE CAMBRIAN ROCKS. 



and are traversed by lines of slaty cleavage also dipping northwards, but at an angle of 45°. In 

 some parts of the quarry, indeed, the surface of the beds and the planes of slaty cleavage almost 

 coincide. Great perpendicular joints cross these slates from north to south, and there are also many 

 backs or joints coinciding with the strike, thus indicating both dip and strike joints. These two 

 sets of joints and the planes of cleavage give rise to rectangular prisms terminated by oblique planes. 

 Veins of calcareous spar mark the course of many of the joints, and one calcareous band of about 

 half a foot in thickness is parallel to the beds. 



We cannot take leave of the slaty rocks of Caermarthenshire without reminding the 

 reader, that although some of them in this district may be the equivalents of the Long- 

 mynd, or mineral axis of Salop, they are here separated from the Silurian System 

 by perishable schists, with beds of calcareous slate, grit, and sandstone occasionally 

 containing fossils. This intermediate member of the series, though made up of 

 the same ingredients as the slates, is seldom affected by a complete slaty cleavage. It 

 is further worthy of remark, that the very few fossils which occur in the intermediate 

 zone are never detected in the fine shale, though we might suppose its structure to 

 have been favourable to the conservation of organic remains. The beds containing 

 fossils are those hard grits or sandstones which reappear at wide intervals in the schist. 

 In short, the zone intermediate between the Lower Silurian Rocks and the slates, is 

 scarcely to be distinguished in external appearance from many of the districts in which 

 the incoherent beds of the Silurian rocks are most developed. The strata in both cases 

 decompose to mud; but in agricultural value the beds of the Silurian System have a 

 manifest superiority, containing usually calcareous matter, whilst its prevalent absence 

 in the upper part of the Cambrian System renders the soil comparatively cold and 

 sterile. The perishable nature of the schist (" rotch") throughout such large tracts, 

 and the absence of bands of hard sandstone by which it might have been sustained, 

 have led to their extensive denudation, and has necessarily obscured in most places the 

 passage from the Silurian into the Cambrian System. 



Of the slaty impress, much has been said to confirm the view of Professor Sedgwick 

 that it has been superinduced upon the materials of which the strata are composed, long 

 after their accumulation. We further learn from observation in Caermarthenshire, 

 that this impress has been strongly communicated in some parts, and suddenly cut off 

 or omitted in others ; and lastly, we perceive, that as it has been repeated at distant 

 parts in rocks of different ages, it is no test of the antiquity of the rock affected, though 

 it may be inferred, that the older slates having undergone a greater number of metamor- 

 phoses, may thus (ceteris paribus) have been rendered more crystalline than the 

 younger. Let us hope that the day is not distant, when the experimental chemist, 

 whether employing heat, electricity, or both these great causes of change, shall reveal 

 to us the method by which the fine silt from which these schistose masses were derived, 

 was converted into the slaty form ; till then, we fear that field geologists must remain 

 satisfied with establishing the actual condition of the subject. 



