396 PREHISTORIC EUROPE. 



general colour is a dull brown or dirty gray ; in some places 

 inclining to pale yellowish gray, and grayish blue. 



Taking all these facts into consideration, we arrive at the 

 conclusion that the Carse-clays are estuarine deposits, and were 

 accumulated at a time when the Firth of Tay reached consider- 

 ably farther inland than it does now. The upper margin of the 

 Carse-clays represents the old water-level, which stood then 45 

 feet above the present mean-tide. After the ancient forests had 

 flourished for a long period, the sea began slowly to gain upon 

 the land, and the ancient Earn and Tay, which, during the age 

 of forest-growth, were probably streams of no greater size, and 

 perhaps were even smaller than the present rivers, acquired a 

 larger volume, and commenced to overflow the low-lying tree- 

 covered plains. Sheets of gravel and sand, and alluvial silt and 

 mud, were thus gradually spread over the site of the ancient 

 forests ; while in some places all relics of the old land-surface 

 were swept away. The torrential character which the rivers 

 assumed at this time is shown by the masses of coarse shingle 

 which they carried along. The trees which grew close to the 

 water were often undermined, and, falling into the streams, 

 were floated away down the valley. Ere long, however, the 

 lower reaches of the valleys were converted into a broad estuary, 

 and the destruction of the forest-bed in those regions was arrested. 

 The muddy rivers still continued to flow with undiminished 

 volume, but the coarser sediment they swept along was arrested 

 soon after it entered the estuary — only the finer mud being- 

 carried farther, and distributed over the whole wide surface of 

 the drowned low grounds. 



The presence of the isolated stones and boulders which occur 

 at rare intervals in the Carse-clays shows that floating-ice was 

 not unknown at this late period. The stones may have been 

 frozen into shore- ice, forming in winter along the margins of the 

 estuary, or they may have been carried down by river-ice. The 

 largest boulder I have observed, however, hardly could have 

 been derived from any region north of Perth. It is a fragment 

 of porphyrite of precisely the same character as the rock of 



