528 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 24, 



far up on these hillsides, must once have been the old high- water- 

 mark. 



I shall now mention some other circumstances that tend to throw 

 light upon the depth of the sea during a part at least of the Pleisto- 

 cene epoch. The evidence of the Mollusca, so far as it goes, indicates 

 that, at the period of their existence, the depth of the sea was not 

 beyond a few hundred feet, even over the lowest part of the present 

 land ; and it is important to bear in mind that their remains have 

 not been found anywhere in this part of the country at an elevation 

 exceeding 250 feet. What I am now going to allude to, however, 

 is the occurrence of beds of gravel and water-worn pebbles on the 

 summits of some of the hills in this eastern part of Aberdeenshire. 



I have shown that the top of the ridge about Highlaw, at an 

 elevation close upon ,300 feet, is occupied by gravel ; there is also a 

 ridge in the neighbourhood of Ellon, called Cross-stone, the top of 

 which consists of great masses of water-worn pebbles and gravel at 

 an elevation of 226 feet. The Hill of Auchleuchries, in the parish 

 of Cruden, has also a crest of gravel, reaching a height of 356 feet. 



Again, there is a ridge of hills which, commencing at the Buchan 

 Ness, runs inland for seven or eight miles, rising gradually till it cul- 

 minates, near its western extremity, in the Hill of Smallburn, at an 

 elevation of 464 feet above the mean level of the sea. This ridge is 

 remarkable for an abundance of flints, which are found along its 

 summit from one end to the other. They are always water-worn, 

 and sometimes contain chalk-fossils. Associated with them is a 

 similar abundance of quartzose pebbles, which are also finely rounded 

 and water-rolled. 



Both the flints and quartz-pebbles extend a little further west, to 

 the hills of Skelmuir and Dudwick ; but towards the western extre- 

 mity of the latter the flints gradually die away, and finally cease. 



These chalk-flints appear to have been first brought under notice by 

 Dr. Knight of Marischal College, Aberdeen, who read a paper upon 

 them to the meeting of the British Association, at Edinburgh, in 

 1 834, and exhibited a series of specimens ; and they have since then 

 been commented upon by various observers*. Their abundance is 

 in many places surprising ; but they are often hidden by a covering 

 of peat-moss. In some places the flints predominate, in others the 

 quartz ; but they are both highly water-worn, the hard quartzy pebbles 

 being often as smooth as an egg, with a clean washed aspect, like 

 the pebbles on the beach. I found that both kinds occur in a thin 

 stratum of reddish or yellowish- brown earth or earthy clay ; and in 

 some places the pebbles form almost the entire mass, as is very 

 observable in several pits at the side of a road which crosses the 

 ridge between Tarhenry and Smallburn (fig. 5). This earthy clay 

 appears to repose immediately upon the subjacent rock. I have seen 

 it covering the granite at the Blackhill, and gneiss at Smallburn, and 



* See a Memoir by Messrs. Salter and Ferguson on this subject, Quart. Journ. 

 Geol. Soe. vol. xiii. pp. 83, &c. — Edit. 



