¥” = 
OLD RED SANDSTONE OF WESTERN EUROPE. 4321 
it presents in many sections on the west side of the island truly intrusive 
characters. Sometimes the mass ‘is seen overlying the sandstone, then it may 
be observed creeping across the edges of the strata and cutting out por- 
tions of them until it comes to rest directly upon the underlying diabase- 
porphyrite. 
At one point, a northern promontory called the Horn of Papa, an arched 
mass of this porphyry, projects into the sea, and is covered by nearly flat felspathic 
v 
VV V/ \ as er S54 ae wait 
ji LN —— 4 




SSS 
= == = 



















Mig. 12.—Section on north-west side of Papa Stour. P. Diabase sheets a contemporaneous lavas). 
S. ‘Sandstones and conglomerates. F. Pink and yellow porphyry or ‘‘porphyréid.” f. f. faults, 
sandstones. I could not satisfy sree: whether these strata were later than 
the igneous rock, and deposited upon its denuded surface, or belonged to the 
same series as the strata below, but now disjoined and borne up from them 
by the intrusive mass. 
In Papa Stour, therefore, we have clear evidence of the eruption of slaggy 
diabasic lavas, the formation of sandstones and conglomerates, partly from the 
eroded surfaces of these lavas, and partly, perhaps, from fragmentary volcanic 
ejections, and the subsequent invasion of all these rocks by a curious tuff-like 
porphyry. That these rocks belong to the Old Red Sandstone system hardly 
admits of question. ‘ The sandstones and flagstones, where free from volcanic 
débris, are so closely repetitions of the Bressay beds on the east side of the 
Mainland, that we may regard them as belonging, not only to the Old Red 
Sandstone, but to the same part of the system as some portion of the sandstones 
and flagstones on the eastern coast. 
The volcanic activity of that ancient period was not limited, however, to the 
eruptions at Papa Stour. On the northern side of St Magnus Bay, according 
to Dr Hipsert, masses of “claystone,” “amygdaloid,” sandstone, and con- 
glomerate occur. From his description, I have little doubt that these rocks are 
a repetition of those of which I have just given an account. But I had not an 
Opportunity of examining them. Dr Hippert’s narrative is strictly geognostical, 
and affords no clue to the geological structure of the ground. Dr HEpp Le 
informs me that some of the rocks are “ trappean breccias or conglomerates,” 
A specimen which he kindly sent to me shows under the microscope the same 
Streaky character as occurs in the mass of Papa Stour. It contains scattered 
fragments, and is no doubt a tuff.* 
* After the visit to Shetland, of which the results are given above, my former pupil, Dr Gzorcr 
VOL. XXVIII. PART II. DR 
