830 REPORT—1885. 
together, there may be two or more successive ‘refractions’ of small 
amount (fig. 8), and if the texture of the foreign bed change gradually 
upward, so as to pass into the 
overlying slate rock, the cleavage 
surfaces assume a curved form 
as in fig. 9. 
The following rules seem to 
hold good in all such deviations 
of the cleavage surfaces :— 
(i.) In passing from slate 
rock to a gritty band, or from a 
finer gritty band to a coarser, 
age the cleavage planes are invari- 
ably bent so as to make a higher 
Fic. 7. 
ge angle with the bedding. 
Bi (ii) The deviation of the 
cleavage planes is of greatest 
amount, ceteris paribus, when 
they cut the bedding at a mode- 
rate angle. If perpendicular, 
there is no deviation, and if the 
angle be very small, the cleavage 
produced in the gritty band is ofa very rudimentary character, the band, 
Fic. 9. 
Fia. 8. 
if thin, splitting in an irregular manner mainly in the direction of the 
bedding.! 
1 Professor Hughes has observed that in cleaved flagstones, ‘ when the cleavage- 
planes approach within about 15° of the stratification, the rock is apt to split along 
the lines of bedding.’ [Lyell’s Student’s Elements, 3rd ed., p. 590 (1878).] In the 
Lingula flags of the Nantlle valley and of the Portmadoc district, it is easy to find 
wedge-shaped ‘junction-specimens’ of slate and flags, having opposite faces slightly 
muelineds one being a cleayage-plane of the slate, and the other a bedding-plane of the 
ags. * ee 
Good slate-rock will split truly along the cleayage-planes, however near these 
may be to the bedding. 
