780 B. N. PEACH AND J. HORNE ON THE 
to work out the succession of the representatives of the Old Red 
Sandstone as developed on the Mainland, as well as the relations of 
the associated contemporaneous and intrusive igneous rocks, on 
account of the important evidence which they furnish regarding the 
ice-movement. While pursuing this object, we were fortunate 
enough to discover in the Walls district a rich series of plant- 
remains in rocks which have been hitherto considered as forming part 
of the metamorphic series. The general character and physical re- 
lations of these altered rocks will be briefly described in a subsequent 
page (p. 785). 
Il. Generar Disrrisution oF THE Rock-¥oRMATIONS. 
As the distribution of the rock-formations has an important bear- 
ing on the question of the glaciation, it will be desirable to give a 
brief outline of the nature and respective limits of the various for- 
mations, so far as these have been already determined. The stra- 
tified rocks belong to two periods :—(a) the Old Red Sandstone; (6) the 
great series of metamorphic crystalline rocks on which the represen- 
tatives of the Old Red Sandstone rest unconformably. To what 
precise part of the crystalline rocks of the Highlands the metamor- 
phic series of the Mainland and the north isles belongs, we do not 
- at present presume to say*. 
There are also associated with the metamorphic series some intru- 
sive igneous rocks, and certain masses which may be viewed as pro- 
ducts of extreme metamorphism. These may probably be relegated 
to the time when the metamorphism of the ancient stratified rocks 
took place. At least some of the igneous rocks now referred to must 
be older than the basement breccias of the Old Red Sandstone, 
inasmuch as the latter in certain localities are composed of angular 
fragments of the former. 
But, further, there are abundant proofs of volcanic activity during 
the Old-Red-Sandstone period, as is evident from the great development 
of contemporaneous and intrusive igneous rocks on the Mainland. 
Similar phenomena are met with in the isles of Papa Stour, Bressay, 
Noss, the Holm of Melby, and Meikle Rooe; but the magnificent 
sections on the western shores of Northmayine justify the conclusion 
that the proofs of volcanic activity on the Mainland surpass in gran- 
deur and extent those of the other Shetland islands. 
The Metamorphic Serves. 
On the Mainland these may be grouped in two divisions, which 
are clearly marked off from each other by distinct lithological cha- 
racters. 
(a) Dark blue, green, and grey schists and clay-slates, with bands 
of quartzite and limestones. 
* For detailed descriptions of the lithological varieties of the metamorphic 
series, see Hibbert’s admirable volume on ‘The Shetland Isles,’ published 1822 , 
also a series of valuable papers by Professor Heddle on “The Mineralogy of 
Shetland,” Mineralog. Mag. vol. ii. pp. 12, 106, & 155. 
