GLACIAL A>'D POST-GLACIAL DEPOSITS. 
177 
The formation of the gravel terraces of Glen Roy appears to have 
taken place in the last half of this epoch — when the land was sub- 
siding. Had they existed previously, they must have been obliterated 
by glaciation. Nothing afibrds a better explanation of their origin 
than the view originally advanced by Agassiz, and lately adopted by 
Lyell, that they were formed in a valley converted into a lake by the 
damming up of its mouth with a glacier which descended from Ben 
Nevis. Admitting that the Glen Koy terraces have been formed in 
this way, it must be conceded that the laud was at the time gradually 
yet intermittently subsiding. "While the subsidence was going on, it 
may be safely assumed tliat the glacier-dam became more and more 
reduced in height, causing at the same time the level of the lake, 
water to fall lower and lower, until, finally, both glacier and lake 
disappeared altogether. Possibly the melting of the glacier was 
facilitated by the gradual setting in of the more genial climatic con- 
ditions which prevailed in the succeeding epoch. It is unnecessary 
to suppose that the land, when subsiding, descended beneath its 
present level in the last stages of the lake and glacier. 
The way in which the glacier-lake of the Miirjclen-see is formed 
and sustained, that is, by the glacier of Aletsch, as described by 
Eamsay, may be accepted as a good illustration, on a small scale, ot 
the origin of the old glacier-lake of Glen Ivoy. 
Post-GlaciaJ Period. 
Pirst epoch (subaqueous). — There are several mountain bogs in the 
west of Ireland containing the remains of rooted trees which could not, 
under present conditions of temperature, grow at their present eleva- 
tion. I am strongly inclined to believe that they grew at a lower 
level. The " Belfast Thracia convexa clay " may have been deposited 
about the same time in deepish water. 
Second epoch (subaerial). — During this epoch, considerable portions 
of the submarine area between the south of Ireland and the coast 
of Spain, and averaging about 200 feet in depth, was elevated above 
the level of the sea, giving rise to a land-surface which botanically 
connected these two countries. The connection, although now phy- 
sically severed, is still maintained, as Forbes noticed in 1846, by the 
presence (*f Asturian plants — Daheocia polifolia, PingiiicuJa grandi- 
flora, Arahis ciliata, Tricliomanes radicans, etc. — in Kerry and Con- 
nemara. Forbes conceived that the " destruction of the intermediate 
land took place before the Glacial period ;"* but since he wrote, the 
researches of Ramsay, Prestwich, Lyell, Trimmer, Godwin-Austen, 
Jamieson, Falconer, Pengelly, Chambers, and numerous others, in Post- 
Pliocene geology, render it highly probable that the " destruction " 
occurred at a much later period ; in short, that Ireland, England, and 
Spain were united during the earliest age of tin-mining in Cornwall. 
In numerous places on the Irish coast there occur extensive sub- 
marine peat-bogs, which, it is evident, cannot have been formed when 
the forests last noticed were growing as supposed, or at the present 
* Mem. Geol. Survey, vol. i. p. 348. 
TOL. VI. * 2 A 
