TYLOE QTJATERIS^AKY aPvATELS. 59 



rather to pluvial than to fluvial action. Of marine remains in the 

 Thames-valley gravels there are no traces. 



In the south and east of England the hills are rarely higher than 

 1000 feet, and the valleys are deep and wide, and are adapted for 

 rivers 20 times as large (or more) as those occupied hy them now. 

 The banks of the old rapid and large rivers are now separated from 

 the banks of the present small streams by a great extent of i^emanle 

 alluvium ; and modern gravel has accumulated in the old river- chan- 

 nels, owing to the decreased velocity of the present streams. 



This is well shown in the large flat plains of gravel through which 

 the Lea river flows, where the river, even in floods, only reaches the 

 bottom of the 30-feet bank at Clapton, while the brick-earths and 

 gravels of the ancient Lea reach to 80 feet above the level of the 

 present stream. 



Some modern streams have still force enough to cut horse-shoe 

 bends and new courses out of the flat plains which were probably in 

 flood-times constantly overflowed by the old rivers running in the 

 gravel-period; but the covering of recent alluvium in the river- 

 channel prevents any observation of the character of the gravel 

 itself. 



Large and deep side-valleys open into the Thames valley, contain- 

 ing small streams or rivers ; but I do not know of any instance of a 

 stream in this district having risen high enough, or had force enough, 

 to cut through any of the Quaternary deposits, so as to expose a good 

 natural section. 



The absence of such natural sections is of important geological 

 interest ; for it is evidence that no marked changes of level occurred 

 during the gravel-period. If elevations of the land had occurred 

 during the latter part of the gravel-period, as has been supposed, and 

 rivers had cut down these beds 30 or 40 feet, then all the side-streams 

 would have had to adjust their beds to the new circumstances of level, 

 and we should have had natural sections in every side-valley, as well 

 as in the Thames valley itself. There are no waterfalls in the district ; 

 and every stream reaches the main river by a very gradual course in 

 obedience to a definite law. 



The fact that no good natural sections of this kind exist in the 

 Thames valley, or in the valleys of its tributaries, is a proof that neither 

 the principal river nor its confluent streams had force enough to 

 break through the upper gravel and to expose the brick-earth and 

 sands below. It is also evident from the same facts that the change 

 in rainfall, and consequent decrease in volume and speed of all rivers 

 and streams in the district, occurred immediately after the close of 

 the gravel-period. 



We find natural sections of gravels along the water- courses of the 

 streams and rivers of South AYales, where the rainfall is still nearly 

 40 inches, and the side-streams fall with great rapidity to the main 

 rivers from the high land. The materials of these gravels are essen- 

 tially different from those of the south and east of England ; and 

 although the two sets of gravels may be identical in age, there would 

 be as much relative diff'erence between the deposits in high and low 



