14 



an earlier jnrt of the recent or alluvial age. 



Tliis fiict which will lie treattnl in tlie concliuliii'; chaiiter of this memoir, 

 lesids IIS to consider thi- ditTereuc*; exhihited by the alluvial se<limeiitj* accortlinj» 

 to their age. It may be said, liowover, that a distiiictioii l»etween t>lder and 

 younger deiMisits of this kind cannot be made everywhere — jiwt as we have see» 

 already in the introduction that «m the 81o|k« of the inmintains encircling tho 

 jilain of Tokio we cannot dniw a cx-rt^iin line even Itetween the two main divisions 

 of the quaternary age, the diluvial and alluvial fonuation. Indee<l tlie distinction 

 of old and new alluvial de|)osit*i is confined to broader river-vallevK, wlienever 

 they show the iilienomeiiou of Urracen, slight us their height and im|i<irtancc 

 may be. By examining our river-lieds, we sliall very often recognize such 

 terraces, and it would be perfectly erroneouR if we sliould confine that nanic to 

 the very large, nay gigantic terraces of some of the river-valley.'« of Nortli 

 America. In any |«rt of the European continent, for instance, we find ternucs 

 along the river-banks of a few metres of lieiglit, anil this seems to bo indeed tlie 

 rule, wliilst those very high terraces apjiarently have an exceptional character. 



The phenomenon is moreover in jierfect conformity with all the laws which 

 concern the action of water, erosion &c, the water of any river always tending 

 to cut into the soil and to flatten and lower the paraliolic line which an a rule 

 is formed by any freshwater-course. Wherever the land is rising, and the 

 level of the upper part of a river-course is raised al)ove the level of the mouth, 

 of course the phenomenon is rendered more obvious. I have already mentioned, 

 and shall recur to this fact, that such is the case in Japan, or at least in the 

 environs of Tokio. We may not Ik? astonishe«!, thereforii, to find terraces along 

 many of the larger riven? mentioned in the first chapter, among which the 

 Tonegawa may be said to lie a good example, although terrace« are formed 

 along the banks of almost all the rivers of the Tokio jilain. 



Of course the deep incisions made by the rivers in the middle and upjier 

 part of their valleys do not strictly, at least not exclusively, l>elong to the 

 formation of terraces, and this is easily seen from the very slow but unvarying 

 deepening of the river-beds themselves within the broa<ler parts of tlie valleys. 

 We cannot deny, however, that a j»art of this action at least Ix-longs to the 

 terrace-jieriod. Most likely in the iipiier jiarts of the valleys this action WgJin 

 already at a time anterior to the alluvial era, continued during all its earlier 

 part, or during the terrace-era, and continues still during tho later jiart of the 

 alluvial age. 



In such cases where we may trace the limits of the terrace formation, we shall 

 be very often aware of lithological, or mineralogical, differences Ixitween its 

 dejKisitB and those anterior and subsequent to it. During the diluvial age 

 conglomerates are of frequent occurrence — ;« the following chapter will alsf» 

 elucidate — alternating with sands and clays. This is sometime«, but comi«ratively 

 seldom, the case also in the terrace-formation, in which *andi are more common. 

 It diifeiB by this character also from the more recent dej>osit8 which are richer 



