266 J. #. Marr—Origin of Archean Rocks. 
were deposited. This irregularity might be due to the inequality of 
earth-movements at the commencement of Cambrian times, a thing 
rendered extremely unlikely by the great uniformity of character of 
many Cambrian deposits over wide areas, and its improbability may 
also be judged of, by attempting to apply this explanation to two 
particular cases which may be taken from many others by way of 
illustration. The first is the mode of occurrence of the Cambrian 
rocks of North Wales and the East Coast of Ireland, as shown in 
Fig. 1, which indicates the manner in which the Cambrian deposits 
overlap one another against the old Pre-Cambrian ridge. 
Gaels 
W. 1D 
g a 
3 ay 5 = 
dp) ) GH S 
a op zg 2 
= fo) Fax} = 
| 
= xq O 4 
A. Archean Ridge. 
5. Arenig Beds. 4. Tremadoc Slates. 3. Lingula Flags. 2. Harlech Series. 
The dotted part indicates shore deposits. Vertical scale much exaggerated, and part W. of 
Carnarvon much shortened horizontally. 
In tracing the Cambrian beds from Llanberis to Carnarvon, a 
thickness of several thousand feet of rock is found to thin out and 
disappear in a horizontal distance of about eight miles, as shown by 
Sir A. Ramsay (Mem. Geol. Survey, vol. iii.), and the same thing 
is seen in tracing the rocks from Ireland towards Carnarvon, only 
the thinning out here takes place more slowly. The inequality of 
movement would have to be very great if this distribution of the 
beds is to be explained in that way, whilst the Llanberis and Irish 
areas were depressed, the narrow ridge between would be kept 
continuously above water. A similar change is shown to occur 
between the Longmynd Hills and the Wellington area, where the 
Hollybush sandstone rests on the Archean rocks, and other phe- 
nomena of the same nature are found in Sweden. They appear to 
point to the submergence of great mountain masses sculptured by 
subaerial denudation, and this would also account for the remark- 
able character of the boulder conglomerates found in Cambrian beds 
of different ages, where they rest upon the Archean rocks, as those 
described by Dr. A. Geikie in the north of Scotland (ef. “ Geological 
Sketches at Home and Abroad’’), and by Prof. Lapworth in the 
Girvan district (Quart. Journ. Geol. Soc. vol. xxxviii. p. 587). 
The high mountains would give rise to glaciers and Boulder-clays 
formed upon their flanks; upon the submergence of these tracts, 
the clays would be re-sorted, and the boulders somewhat rounded. 
Some such explanation as this is required to account for the great 
size of the stones in many Cambrian conglomerates of different ages, 
