552 Proceedings of the Royal Society 
These markings were particularly visible in the gorge of the 
estuary at Stirling, and up to a level of above 350 feet. 
7. Attention was next called to the boulder-clay of the district, 
whose irregularity, both of distribution and of thickness, was 
pointed out. 
The fact of marine beds found above, below, and in the heart of 
the boulder-clay was noticed, as also the force with which it appeared 
to have been pushed by some agent or agents in an easterly direc- 
tion. 
The colour of the boulder- clay, varying as it does in different 
districts, seemed to be owing to the colour of the rocks situated in 
each case to the westward, and it was observed that beds of strati- 
fied or laminated clay sometimes showed different colours, concur- 
rently with the boulder-clay. 
The author next adverted to the two theories proposed to explain 
the origin of the boulder clay, the one being that it was mud 
formed by a glacier grinding the rocks, over which the glacier 
flowed, and the other that it was formed i>y icebergs ploughing 
through and mixing up a previously existing sea-bottom composed 
of mud, sand, clay, gravel, and boulders. He stated that he adopted 
the second view, and specified some of the difficulties of the ques- 
tion, all of which he thought could be more completely solved by 
the iceberg than by the glacier hypothesis. 
8. The author then adverted to the subject of ancient sea- 
margins, four or five of which he specified, mentioning the names 
of places where they had been observed by himself or others. 
The lowest of these sea- margins he had drawn on a map, which he 
exhibited, and the height of this margin he stated was, at the head 
of the old estuary (west of Stirling), about 30 feet above the same 
old sea-margin at Dunbar. This rise he attributed to tidal action 
exclusively, pointing out that even now, when the estuary is so 
much shallower, the sea-level at Alloa is 6 J feet higher than at 
Dunbar, and that there are other estuaries, in this country and 
others, where now the sea-level is at their head more than 30 feet 
above the level at their mouth. 
9. The author said that the change in the relative levels of sea 
and land, which these ancient sea- margins indicated, might have 
been brought about either by the land rising or by the sea sinking; 
