Mineralogy and Geology. 431 
One law respecting slaty cleavage was announced in 1831 by Pro- 
fessor Sedgwick,* and is now well known: that law is, that the cleay- 
more constant and regular than the strike of the beds 
order to present, in a succinct form, the evidence from which the 
author has deduced this second law respecting slaty cleavage, the ob- 
servations he made, in various parts of North Wales, of the positions 
southeast, and the cleavage planes are nearly vertical. On the east side 
of the anticlinal, the beds dip southeast, and the cleavage dips northwest 
to 65°... On the eastern side of the Carnarvonshire synclinal, 
the beds dip northwest, as does also the cleavage, but at an angle whic 
geduelly diminishes as you recede from the Snowdon chain. Thus at 
@ Rhiw Brefder quarries the angle is 55° ; at the Diflwys quarries it Is 
45°, and at Manodmawr 35°. Towards the northern extremity of the 
‘N.E., as does also the strike of the cleavage. To the south of Tre- 
Madoc the beds change in strike from northeast to east, and the cleay- 
ge changes in strike from northeast to E.S.E. 
_ The parallelism in the strike of the planes of bedding and cleavage 
approaches to east; but it is subject to many local yariations; and in 
* Geol. Trans.. ii Ser.. vol. iii 68. 
‘+t While the author was seiwinglble conclusion from his observations in Wales, 
nearly similar law was anno to the British Association at Cork by Professor 
i The cleavage planes of the slate rocks of 
