310 Hutt—On the Geological Age of the North Atlantic Ocean. 
are altered sediments laid down on the floor of the primeval ocean, I may be 
allowed to ask, where, if not in the region of the Atlantic Ocean itself, are we 
to look for the source of these sediments—the pre-Laurentian lands in fact—from 
the waste of which these great deposits were themselves constructed ? and if this 
be a fair inference, it results tnat, since pre-Cambrian times, the present distribu- 
tion of land and water, as far as these continents and the intermediate ocean are 
concerned, has been in the main reversed.* 
(2) The Silurian Group.—The Silurian rocks occupy large areas, both of the North 
American continent on the one hand, and of the British Isles and Western Europe 
on the other—areas which are largely concealed beneath newer formations, and 
which, previous to denudation, must have left but very small portions of the lands 
on either side of the North Atlantic uncovered (Plate VIL., fig. 2). 
The evidence for the position of the land-surfaces of the Silurian, and 
particularly the Lower Silurian, period may be thus stated :— 
(a) British and European Area.—The Silurian areas, visible and concealed, occupy 
nearly the whole of the British Isles, with the exception of the North-western 
Highlands of Scotland and Ireland, and the granite tracts, which are due to 
subsequent igneous (or metamorphic) action and erosion. We are led also to infer, 
from the position of the strata towards the west of the existing areas, that origi- 
nally no part of the British Isles was left uncovered by strata of the Lower 
Silurian period, and that the waters of the sea in which these beds were deposited 
occupied the whole region of these islands. (See note at end, p. 318.) 
On the other hand, the predominance of quartzite and siliceous strata in the 
North-west Highlands of Scotland and Ireland points to the presence of granitic 
or gneissic lands in that part of the Atlantic lying to the west and north of the 
submerged tract. 
In the above observations I have assumed that the crystalline strata of the 
North Highlands of Scotland are of the geological age originally determined 
by Sir R. I. Murchison, of the truth of which conclusion I had satisfied myself not 
only by the study of his writings, and those of other geologists, but by personal 
examination of the Northern Highlands during the spring of 1880. Whatever 
modifications more recent researches may have required will not invalidate my 
argument. (See note at end, p. 318.) 
* The ‘* Geological Magazine”’ for June, 1883, contains an interesting article by Mr. J. E. Marr, on 
the origin of the Archean Rocks, in which he suggests their volcanic origin, with subsequent metamor- 
phism. I doubt if he is likely to get many converts to this view. The Archean or Laurentian strata differ 
very little from some of the more highly metamorphosed beds of Lower Silurian age. Another article» 
by Mr. W. O. Crosby, deals very ably with the views of Prof. Le Conte, Prof. Dana, and Mr. Wallace, 
regarding the origin of continents and oceans. 
+ The presumable position of land and sea during the Lower Silurian period is represented in 
Plate II. of ‘* Physical History of the British Isles’ (1883). 
