THE ISLAND OF FOULA. 
241 
jecture, belongs to the unstratified portion to which it is 
opposite, and to which, I imagine, it owes its ability to re- 
sist the action of the waves. 
This occasional interruption of the stratification of a rock 
in the line of its direction, is a circumstance that deserves 
to be well studied. A most remarkable instance of the 
same kind occurs near Lambaness, in the Island of Stronsa. 
Among the lower beds of the sandstone, a few thin layers 
of limestone occur, very compact, of a blue colour, and re- 
sembling siliceous schistus; argillaceous matter also, in 
combination with the mica in very thin lamina, serve occa- 
sionally to give the sandstone a schistose structure. But 
the circumstance most worthy of attention, is the occurrence 
of some thin beds of indurated clay, containing minute 
scales of mica ; these are from a foot to two inches in thick- 
ness. The sandstone in the immediate vicinity has much the 
appearance of quartz-rock, and fills up numerous rents and 
openings of the indurated clay, which is chiefly separated 
by openings vertical to the plane of stratification, present- 
ing appearances similar to what clay assumes in drying. 
The following sketch of a small portion will convey a no- 
tion of the manner in which it occurs, in which the white 
spaces represent the sandstone with its branches alternating 
with and traversing the clay, which forms the black parts of 
the figure. 
VOL. IV. 
Q 
