134? PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16, 



all the rocks in the uncoloured portions of Sir R. I. Murchison's 

 Silurian Map ; indeed all the older slate rocks of North and South 

 Wales beyond the limits of the Silurian system, as well as all the 

 older slate groups of the Cumbrian mountains. 



2. I shall endeavour to give an approximate view of the suc- 

 cessive older groups in South Wales, so as to put these groups in 

 approximate coordination with those of North Wales. 



3. I wish to explain the evidence on which we may approximate 

 to the thickness and natural grouping of the successive Cambrian 

 deposits, so as to put them in their true relation to the lower groups 

 of the Silurian system, as published by its author. 



4. From a review of the leading facts, physical and zoological, and 

 a comparison of them with the development of a contemporaneous 

 series in the Cumbrian mountains, I shall attempt to point out the 

 true classification and nomenclature of the whole series — from the 

 oldest Cambrian slates to the newest Silurian groups, ending with 

 the Old red sandstone. To discuss these several subjects fully would 

 require a volume : what I have to offer is a bare outline. I pur- 

 pose, however, to take up these questions in subsequent, and, I hope, 

 early, communications. 



For the present I must defer any discussion on the phaenomena 

 and theory of cleavage-planes. But I may remark by the way (as 

 my published views have been misrepresented or misunderstood), 

 that I considered cleavage-planes as the resultants of all the polar or 

 crystalline forces simultaneously affecting the rocks exhibiting clea- 

 vage in any given mountain chain. Consistently with this view, I 

 supposed that even unstratified rocks might exhibit a kind of clea- 

 vage ; and from the analogy of slate rocks (where the prevailing 

 strike of the cleavage-planes nearly coincides with the strike of the 

 beds, as described in my published paper), as well as from theore- 

 tical views, I stated that this cleavage would probably be parallel 

 to the axis of the chain*. Of this arrangement I gave one or two 

 examples from the granitic chain of Cornwall. So far from over- 

 looking the fact, that in large masses of slate the cleavage-planes 

 strike nearly with the beds, I pushed the generalization too far. 

 For during the latter years of my surveys in Cumberland (though I 

 could not but observe that there were many exceptions to the rule in 

 the less perfect, and more disturbed, slate rocks of Westmoreland), 

 I assumed, during all my traverses among the higher mountains, that 

 the strike of the cleavage-planes was identical with that of the true 

 beds. To affirm that cleavage-planes are nearly parallel to the axis 

 of a chain — that they run nearly with the beds — or that they are 

 parallel nearly to the principal anticlinal or synclinal lines — are three 

 identical propositions expressed in different words. But what may be 

 the primary forces of aggregation producing this arrangement, is an 

 entirely distinct question which I am not permitted now to discuss. 



Neither shall I now discuss the nature of the contemporaneous 

 trappean masses which enter so largely into the composition of the 



* Geol. Trans., Second Series, vol. iii. pp. 473, 483. 



