194 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 11, 



the marine clay, at Errol brickwork, full of Arctic shells in situ, and 

 reposing on the irregular surface of the azoic boulder-earth. 



Even in the little valley of the Ythan we have a very complete ex- 

 hibition of the series. Thus, in the section opened up by the railway, 

 where it crosses the valley near Ellon (fig. 2), we have, commencing 

 at the bottom (1st), the ice-worn gneiss covered by the grey glacier- 

 mud with heavy scratched boulders aU brought from the west- 

 ward, and (2nd) reposing on this a mass of fine laminated red 

 marine clay, found to be upwards of 20 feet deep at the foundation 

 of the railway bridge ; (3rd) resting on this clay at the bridge there 

 is a mass of rough valley-gravel 15 feet thick ; while, more recent 

 than aU these, we find at the estuary, fig. 10, (4th) the bed of peat with 

 remains of trees resting on the gravel ; (5th) the raised estuarine 

 mud with shells on the top of this peat; and (6th) covering this 

 raised estuarine silt we have heavy masses of blown sand, old shell- 

 mounds and chipped flints, and in some places a little peat. 



Fig. 10. — /Section showing the relations of the superficial Deposits 

 in the Estuary of the Ythan. 



River Ythan. 



1. Gneiss-rock. 4. Stratum of peat. « 



2. Boulder-earth, or Glacier-mud. 5. Old estuarine beds with shells. 



3. Fine stratified clay and sand, or Glacial-marine beds, 



I therefore infer, from the evidence adduced in this and my former 

 papers, that in Scotland there has been a succession of conditions 

 during the Post-tertiary period somewhat -as follows : — 



1st. After the deposition of the Crag-gravel, and after the Mam- 

 moth had lived in Scotland, the country was covered with a great 

 depth of snow and ice, which must have extingiiished the pre- 

 existing flora and fauna. This ice moved outwards in broad streams 

 from the great watersheds of the country, carrying with it much 

 stony debris and multitudes of boulders, which it left in irregular 

 sheets, constituting the old boulder-clay or " till" of some authors ; 

 the ice also scratched and furrowed the rocks, destroyed the pre- 

 existing alluvium, and exercised a considerable amount of abrasion 

 on the surface of the country. 



2nd. After this state of things had continued for a time, a de- 

 pression of the land took place to the extent of some hundreds of 

 feet, so that all the lower grounds were below the sea-level, but as 

 to the full extent of the depression we are still ignorant. During 

 this submergence the brick-clays containing Arctic shells were de- 

 posited, boulders were drifted here and there by floating ice, and it 

 seems probable that the ice still covered much of the land, and even 

 protruded into the sea along the main valleys in the form of large 

 glacier- streams ; so that the condition of the country would have 

 %een like the present state of Spitzbergen. 



