The Scottish Naturalist. 



341 



side is covered with a bed of boulder clay often 50 feet in thick- 

 ness. 



While the main contents of this clay, however, are derived from 

 rocks at no great distance to the north-west, large boulders of 

 granite, gneiss, basalt, etc., are found, which must have been 

 carried by the moving ice from the higher mountains at or near 

 the central ice-shed of the Grampian range. Seeing that these 

 travelled boulders are found at various depths in the clay they 

 were probably stranded at different periods ; and by carefully 

 noting the depths and positions at which they occur, as well as 

 their petrological character, we might construct a boulder chron- 

 ology, beginning with those nearest the rock and ending with 

 those at the surface of the clay. That many of these large 

 boulders were passed over by the moving ice-sheet after they were 

 stranded is shown by the fact that they are scratched on their 

 upper surface in exactly the same direction from west to east, as the 

 rocks under the clay. A fine example of this may be seen at 

 Sunnyside, where a basalt boulder ten or twelve tons in weight was 

 exposed while lowering the road in 1884. By the aid of a traction 

 engine it was removed from the clay bed where it had rested since 

 the glacial epoch to its present position, great care having been 

 taken to place it in exactly the same position as regards the points 

 of the compass as that in which it was found. 



While speaking of boulders, I may mention that large travelled 

 boulders are often fished out of the depths of the North Sea by 

 trawlers. I have a considerable collection of these sea-boulders, 

 brought ashore by the Messrs. Johnston's trawlers ; most of them 

 are polished and scratched like the land boulders, and present 

 other points of interest. I may suggest to the members of our 

 Union resident at seaports that if they were to use their influence 

 with the owners of trawlers to induce them to bring ashore 

 these marine boulders, noting at the same time the position on a 

 chart where they were found, in what depth of water, their petro- 

 logical character, etc., an important chapter might in the course 

 of time be added to our already considerable and deeply interest- 

 ing history of the Ice Age. 



During the formation of the Boulder-clay the land was sinking; 

 so that what was once dry land became a sea bottom, on which a 

 fine reddish-blue clay, mostly the washing of the inland mountain 

 drifts, was spread. This, which we term the glacial marine clay, 



