470 Prof. Sedgwick, on the Structure of large Mineral Masses. 



imagined (at the time the paper was written) that each great alternating mass 

 of slate had an independent cleavage^ produced probably by crystalline forces 

 actin"- under a high temperature ; and this high temperature seemed to be 

 naturally accounted for by the presence of the porphyries. 



In the great chain of North Wales we have the same indefinite alterna- 

 tions : but the porphyries are less abundant in proportion to the other masses^ 

 and have produced a less impress on the slate system. Some of the slates are 

 crystalline, and some earthy ; and in both varieties we find (though rarely) 

 traces of organic remains. The whole system of slates and tabular porphyries 

 has been thrown into a number of great undulations, producing through the 

 chain a series of longitudinal anticlinal and synclinal lines*. Lastly, parallel 

 lines of cleavage not merely affect given beds ; but sometimes run, without 

 deviation, even through coarse mechanical subordinate strata, affecting whole 

 ranges of mountains, and preserving their parallelism in spite of undulations 

 and anticlinal lines f. These facts have led me to give up the opinion, that 

 the cleavage planes have been materially modified by any action of the alter- 

 nating porphyries. 



Again, among the Cumbrian mountains there is an upper division of slaty 

 rocks, not noticed in the preceding account. It is separated from the green- 

 slate zone by continuous bands of limestone, full of organic remains. It is of 

 much coarser structure than the rocks last described, and has no alternating 

 beds of porphyry ; but it contains some deposits of good roofing-slate, alter- 

 nating with coarse greywacke, and with flaggy beds having no distinct 

 cleavage. 



The same descri})tion applies, almost word for word, to an upper Welsh 

 system, which occupies a great portion of Denbighshire, the chain of the 

 Berwins, and a considerable part of South Wales : but in some of these 

 regions the alternating felspar rocks occasionally reappear, and true slate 

 rocks are more extensively developed than in the corresponding part of the 

 Cumbrian chain. And it is important to remark, that in Radnorshire and 

 Caermarthenshire large tracts of green chloritic roofing-slate reappear in this 

 upper system, without being accompanied by any masses of tabular porphyry, 

 like those associated with the older chloritic slates of Cumbria and North 

 Wales, and without the presence of igneous rocks in any of the neighbouring 

 country. These facts also induced me to modify my former opinions, grounded 

 on the frequent association of the hard chloritic roofing-slates with the tabular 



* See Plate XL VII. fig. 1. 



t See Plate XLVII. fig. 4. and 4 a. 



