50 
and the purple colour more intense near the lower edge. The 
cleavage planes are almost invariably more or less deflected in 
passing through the band, and like the colouring and texture of 
the slate, the deflection is most acute at the lower edge. After 
carefully examining and comparing a large number of specimens 
from various parts of Yew Crag quarry, I was led to conclude, so 
far as I dared to do without microscopic examination, that the 
deflection in the cleavage planes, together with the altered colour 
and texture of the slate, were probably due to vertical pressure, to 
which the bed had yielded subsequently to the period when 
cleavage was developed.* For the purpose of obtaining clearer 
evidence on this point I had some slides prepared from the band, 
and Professor Bonney kindly examined them for me, and gave 
permission for his notes on the slides to be printed as an appendix 
to my paper. Mr. Goodchild also kindly made drawings of two 
of the slides, which are annexed hereto.t The result of Professor 
Bonney’s examination being unfavourable to the theory of meta- 
morphism produced by vertical pressure, it would seem, therefore, 
more probable that the deflection in the cleavage planes has been 
caused by the abrupt passage from very fine to somewhat coarser 
materials. The fine dust of which the red band is composed 
(especially the lower part of it) appears to have been impregnated 
with an unusually large amount of ferric oxide, while there has 
been a corresponding diminution in the quantity of chloritic 
colouring matter which is so prevalent in the adjoining beds. 
Rigg Head quarry has been worked thirty years or more, in the 
same beds as Yew Crags, Honister, and Dubs quarries ; extensive 
excavations have been made and a considerable quantity of slate 
of good quality has been obtained from it. 
* Note by the Editor. When this paper was read I expressed to Mr. 
Postlethwaite my opinion that these dark purple bands were not due to meta- 
morphic action, as he then supposed; but were simply bands of the same 
sedimentary material as that composing the Skidda Slates; and a careful 
examination of the slides under the microscope when drawing them for the 
annexed figures, has confirmed that view. 
++I am greatly indebted to Professor Bonney and Mr. Goodchild for the 
yaluable assistance they have so kindly rendered in these matters. 
