708 Proceedings of the Royal Irish Academy. 
more nortliward, there is either an ordinary fault, a downthrow to the 
northward, or a thrust plane, up which the schistose rocks were over- 
thrust on to the Silurians. In either case the Silurians must he un- 
conformable to the schistose rocks, as the sandstones of the first are 
made up largely, if not solely, of the detritus of the latter. Further- 
more, the metamorphic rocks, both north and soutli of Kilbride Bay, 
are portion ^of one mass of rock ; and as those to the south are ocularly 
unconformable to the Silurians, it follows that those to the north 
must also be unconformable. 
North-west Galway (Yar-Connaught) Metamorphic Rochs. — The 
statements made by Dr. A. Geikie in his Paper published in "Pro. 
Roy. Inst. Gt. Brit.," in connexion with these Galway rocks, given 
in the order in which they occur, are as follows : — 
Loc. cit., p. 529. — " The oldest known rocks of Europe, now 
generally termed Archaean, . . . give rise to topographical features 
which, when fully developed, strongly distinguish them from all 
younger formations. IN'owhere else can such extraordinary unevenness 
of surface be found. Knobs, hummocks, and ridges of almost bare 
rock, separated by narrow gullies or by wider winding valleys, roughen 
the ground in all directions. In the hollows lie innumerable tarns 
and lakes, or flat tracts of bog, where lakes once were. In some 
districts, indeed, there is as much water as land in a given number of 
square miles. 
The statement as to the age of the rocks is so put that hereafter 
the writer may state that his observation only applied to the age of 
the British and Irish rocks, although evidently from what is subse- 
quently stated the writer intends to impress on his readers that these 
Scotch and Irish rocks are the oldest in the world. If such is his 
meaning, the assertion is highly problematical, as none of the rocks 
to which he refers have the characteristics of the Laurentian ; that 
is, of the oldest rocks of America at present recognized. These 
Scotch rocks more probably are equivalents of one of the groups 
in the AlgonUans of the U. S. American Geological Survey, or the 
Ontarians of Lawson of the Canadian Survey."^ 
^ Logan classified the pre-Cambrian rocks as Laurentians and Huronians ; and 
Selwyn on his map (published 1885) in a great measure follows this classification, 
except that the rocks in the ridge extending from Eastern Quebec into Vermont are 
only called pre- Cambrian, Irving classes the pre-Cambrian rocks of the Lake 
