Vol. 54.] IN BOUTH WALES, DEVON, AND CORNWALL . 275 



and angular forms in the tin-ground ; for the difference which has 

 been observed between the tinstone in the tin-ground and that in the 

 local mineral veins ; and for the absence of other ores in the former 

 which are abundant in the latter — difficulties which have always 

 been recognized in speculations on the origin of stream-tin. 



Having regard to the resemblance which the ' tough ground ' at 

 Pentuan as described by Colenso bears to the clay-with-boulders in 

 precisely similar positions at the bottom of some of the valleys round 

 Plymouth, and to the considerations set forth above, I am disposed 

 to think that the stream-tin gravels are rather of Glacial age than 

 that they are more recent than the. raised beaches. 



It will be noticed that the most easterly valley in the English 

 Channel which has been described is that of the Dart, and unfortu- 

 nately there is no information about the rock-bottoms of the 

 rivers farther up the Channel. The foundations of the long bridge 

 over the Teign near Teignmouth might afford an interesting section, 

 but there are no records of them, and of the Exe nothing is known. 



Earther eastward, beyond the shallow West Bay, are the valleys 

 which were once tributaries of the old Solent, and the Thames YaUey. 

 Whatever may be the age of the gravels bordering the former valleys 

 fit high levels, they are certainly in part post-Glacial, and since they 

 were spread out the rivers have scooped out wide and deep valleys. 

 In the Thames Valley the Boulder Clay is at the lowest 90 to 120 

 feet above the sea-level, and the river must have lowered its bed by 

 more than that since the Boulder Clay was deposited. It seems to 

 follow, therefore, that if the valleys which have been under considera- 

 tion are older than the Boulder Clay, they must have existed long 

 before the valleys of the Solent and the Thames were scooped out 

 to anything like their present depth, and when land perhaps drained 

 by tributaries flowing eastward to the Solent filled up part of the 

 English Channel. 



The difference in age, and in the conditions under which they 

 originated, have left their mark on the natural features of the two 

 sets of valleys ; but later, perhaps after the opening of the Straits of 

 Dover, the conditions seem to have become more alike. Raised 

 beaches and submerged forests appear all along the coast, except in 

 the shallow West Bay between Portland and the Exe, and the later 

 alluvial deposits with peat-beds are found in all the valleys from 

 Milford Haven to the Thames. The relative position of these later 

 deposits is, however, different. In the western valleys they overlie 

 the Glacial beds, but in the vallej^s of the Solent and the Thames 

 they lie at a lower level than the Glacial and post-Glacial beds which 

 border the sides of the valleys. In the Thames Valley at Tilbury 

 Docks peat-beds were found at 50 to 60 feet below high-water level, 

 lying on gravel over the Chalk, and the chalk-bottom was as much 

 B,s 74 feet below that level, or 56 feet below the level of low water, 

 in one place. This is, after all, very little deeper than the present 

 river-bottom close by, and may not be the deepest part of the old 

 channel, but it is 160 feet lower than the Boulder Clay at Horn- 



