PRESIDENTIAL ADDRESS. 459 
extended from the North-West Highlands to beyond the Eastern Highland 
border. The Upper Cambrian terrigenous sediments which we now find at 
Stonehaven and Aberfoyle must have been derived from land to the south. In 
Llandeilo time the Arenig and Lower Llandeilo rocks of the Girvan area were 
elevated and subjected to extensive denudation. On this highly eroded plat- 
form, as first proved by Professor Lapworth, coarse conglomerates, composed 
of the underlying materials, were laid down in association with the Stinchar 
and Craighead limestones. In my opinion the appearance of the American 
forms in these limestones is connected with the movement that produced this 
unconformability in the Girvan area. ‘This local elevation was probably asso- 
ciated in some form with the great crustal movements that culminated in the 
overthrusts of the North-West Highlands and caused the intense folding and 
flaser structure of the rocks along the Highland border. By these movements 
shore-lines may have been established between the north side of the old Paleozoic 
7” and the Girvan area, which permitted the southern migration of the American 
orms. 
Note.—Since writing the above my attention has been directed to the recent 
work of Bassler on ‘The Early Paleozoic Bryozoa of the Baltic Provinces,’ pub- 
lished by the Smithsonian Institution in 1911. In his introduction the author 
has shown that the Ordovician (Lower Silurian) and Gothlandian (Upper 
Silurian) rocks of the Baltic Provinces contain a large percentage of bryozoan 
species, in common with the Black River, Trenton, and Niagara limestones of 
the same relative age in Eastern North America. This fact suggests that 
during Lower and Upper Silurian time the old lines of migration were still open, 
and that the Bryozoa, being of clear-water habit, were able to cross the old 
trough from side to side. 
The following Papers and Reports were then read :— 
1. Gn the Geology of the County around Dundee and St. Andrews. 
By Professor T, J. Jesu, M.A., M.D. 
2. A Mull Problem: The Great Tertiary Breccia. By EH. B. Batey. 
[With the permission of the Director of the Geological Survey.] 
There is in eastern Mull a great breccia-formation, with, in places, several 
intercalated rhyolite lavas. The breccia consists for the most part of an 
unbedded assemblage of blunted blocks and fragments of gneiss, granophyre, 
gabbro, and basalt, often associated with rhyolitic débris, for which latter a 
truly volcanic origin can be readily admitted. The problem is whether the non- 
rhyolitic material of the breccia has been derived through explosion or erosion. 
If the former alternative is adopted, many of the breccia outcrops must be 
regarded as marking the sites of volcanic vents, since in several cases the 
boundaries of the breccia are frankly transgressive. At the present time it is 
considered that the evidence favours an alternative view, that the breccia is an 
unconformable formation later than the basalt lavas of Mull, and that its trans- 
gressive relations are due to erosion which preceded and accompanied its 
formation. The basalt lavas of Mull have been violently folded into a series of 
anticlines and synclines, and it is in one of these synclines that the main 
outcrop of breccia in eastern Mull is preserved, with every appearance, more- 
over, of approximate conformity to the surrounding basalts. Here it is diffi- 
cult to escape the conclusion that the breccia is a thick layer overlying the 
basalts and folded with them. Alongside of the syncline is an abrupt anticline, 
in which are exposed all the rocks commonly recognisable as fragments in the 
breccia. The anticline has a core of gneiss, flanked locally by upturned meso- 
zoic sediments, aud these by steeply dipping basalt lavas; intruded, chiefly 
into the gneiss, are granophyre and gabbro. Patches of breccia, distributed 
without reference to geological structure, occur in this anticlinal region, and 
rest upon or against all the rocks mentioned above. Although no positive 
