Greenwood — On Valley Terraces. 205 



geologists appear to have observed the discordancy of their great 

 Pentamerus zone. The fruitful idea in this case was conceived by a 

 good palaeontologist, before he had even seen the rocks in question ! 

 And it was followed up by a man of genius, who had learned the 

 importance of the fossil data to be second, and not first, in geological 

 study. Coming fresh from Wales, where the Silurian rooks are alto- 

 gether unconformable on what he termed Upper Cambrian or Bala 

 rocks, he was ready to make the most of the suggestion of his friend, 

 and was the cause of proof being made by his pupils in the science. 



IV. — ^Valley Tekraces, 



By Colonel George Greenwood. 



(PLATE X.) 



IN the Journal of the G-eological Society for November, 1866, page 

 463, Mr. Tylor gives a paper on " The interval of time which 

 has passed between the formation of the upper and lower valley 

 gravels." He says, "Mr. Prestwich argues that, although the upper 

 valley gravels are at a higher relative level than the lower, yet the 

 higher series are always the older, and the lower the more modern ; 

 and we have thus the ordinary superposition of new over old strata 

 supposed to be reversed. This difficulty is considered by most geo- 

 logists to have been surmounted by Mr. Prestwich's arguments, and 

 his classification of the gravels has been generally accepted. My 

 own opinion is that the evidence on which Mr. Prestwich's theory is 

 based is insufficient." If the gravel terraces were on one another 

 as well as above one another, there would not only be a difficulty, but 

 an impossibility to surmount. But the terraces are not on one another 

 — that is, although they are in level above one another, they are not 

 vertically above one another. If in making a railroad cutting the 

 sides were terraced, the upper terraces would be the first formed, 

 and would consequently be " the older." The lower terraces would 

 be the last formed, and would consequently be " the more modern." 

 Mr. Tylor himself will allow that the upper part of a railroad cut- 

 ting must be excavated before the lower part of it ; and that if de- 

 posits were made during the excavation, the upper deposits would 

 be the first made or " the older ;" the lower deposits would be the 

 last made or " the more modem." The case is the same if, instead 

 of the formation of a railroad cutting by navvies, we suppose the 

 formation of a valley, and the excavators to be rain and rivers. Mr. 

 Tylor feels himself aggrieved that the valley of the Somme has " a 

 transverse section of five hundred times the area of the present 

 water-channel." It might have the same area without any " water- 

 channel " — that is, without any river at all, simply from the erosion of 

 rain. Indeed, it is only when rivers are flooded by rain that they erode 

 the non-alluvial parts of their channels and deposit on the alluvial parts. 

 So in chalk and other porous districts where the downward soakage of 

 rain is not stopped, and consequently, where no springs are forced out 

 to make streams, enormous valleys are formed by the erosion of rain 

 only. Such may be seen in Salisbury plain. I have said, in " Kain and 



