on the Coast of Scotland. 303 
of a quadrangular prism ; hence, if it be an undescribed species, 
which it probably is, it may be denominated E. quadrangularis „ 
On the 4th we left Stromness, which we had reached the pre- 
ceding evening, for the purpose of visiting the curious sandstone 
rocks at Yestnaby, near to Skail. The strata have an inconsi- 
derable dip, and consist of thin slaty sandstone, abounding with 
argillaceous and ferruginous matter, with minute scales of mica. 
The uppermost layer (or even layers), when exposed to the ac- 
tion of the atmosphere, aided occasionally by the spray of the 
sea, becomes divided into numerous tabular pieces, by means of 
vertical rents, and exhibits the appearance of Mosaic work. 
These tables are angular, but differ in the number and relation 
of their angles, although they sometimes exhibit regular geome- 
trical forms. Upon the layer being thus divided, decomposi- 
tion speedily takes place. It proceeds from the sides towards 
the middle, and as the decomposed matter is washed off, the 
table exhibits a depressed border, marking the extent of the de- 
cay, having the middle raised, and consisting of fresh matter. 
In some cases there is a well marked groove separating the two 
portions. In both species there are prominent irregular pieces 
of the rock, which seem little liable to decomposition. In the 
sandstone in the neighbourhood, there are a few inconsiderable 
beds of thin slaty limestome, much impregnated with bitumen 
and pyrites. 
While again entering the Pentland Firth, and at the mouth 
of Longhope, a whale remained a considerable time in the 
neighbourhood of the vessel. It came to the surface several 
times, at a short distance, and enabled us, by perceiving the fin 
upon its back, its plaited breast, and pointed head, to identify 
it as the Balenoptera rostrata. 
( To be continued .) 
Art* XI. — On the order of the Appearance and Progress of 
the Aurora Borealis . By the Reverend James F a rquh ar- 
son,, Alford. (Communicated by the Author.) 
T HE Aurora Borealis is very frequently visible in Aberdeen- 
shire. I have had opportunities for observing it a very great 
