1 32 Notice of Jackson on Geographical Arrangement, ^c. 



Tewkesbury to Aust, receiving no branch thought worthy of a name in the 

 map ; and the Wye runs up to Monmouth, without receiving any branch 

 at all. While many of the smaller rivers, as the Taafe or Parrot, commence 

 their ramifications almost at their mouth, the Nile receives only one tribu- 

 tary below its bifurcation in Ethiopia. These, however, it seems, are to 

 be considered as exceptions, for Colonel Jackson states, though without 

 proving it, that, generally speaking, the largest rivers are those which have 

 most orders of ramifications. 



If, then, there be a natural correspondence between the size of a stream 

 and the orders of its ramifications, one great step towards classification 

 may be considered as gained. 



There remains another, but, in Col. Jackson's opinion, a minor difficulty. 

 How is the main stream to be invariably distinguished from its affluent 

 tributary, or the continued course of a first, from the influx of a rami- 

 fication of the second order ? How are we to determine whether the Tame 

 or the Isis be the continued line of the Thames, or to which of the great 

 branches of the Nile the name of that river should be given ? With respect 

 to the geography of the old world, custom has already decided such points. 

 In Southern America, where are many large streams yet unnamed, we 

 learn that the difficulty is still felt, nor does Col. Jackson, in pointing it 

 out, suggest any remedy. It is obvious that where this difficulty exists, it 

 may cause a stream to be described by one traveller as composed of four 

 or five orders of ramifications, and by another as of only one, according 

 as one or another line be considered as the continuation of the principal 

 stream. 



This difficulty, however, is one that time must gradually diminish, and 

 at no very distant period altogether remove ; so that on the whole. Col. 

 Jackson is of opinion that the classification of rivers by their ramifications, 

 seems to be that system most likely to be generally practicable. 



Col. Jackson divides streams under five classes, since the very largest 

 are not found to possess more than five orders of ramifications. The cur- 

 rent falling immediately into the sea, he calls the primary recipient. 



River islands are divided by Col. Jackson into two classes ; Branch 

 islands, or those formed by the anastamosing branches of a river ; and 

 Channel islands, or such as are not formed by any branching off of the 

 river, but are found in its regular channel. Among branch islands arc 

 those formed in deltas, or deltoid islands. 



Of channel islands we have -6s(/-islands, or those formed of the same 

 material with the bed and banks of the river ; and jBaw^-islands, which arc 

 later deposits, brought down and left by the stream. When such a deposit 

 is below the level of the water, it is called simply a Bank. 



The Bed of a stream is the excavation which it usually occupies, and 

 over which it flows ; the term Channel being limited to its deepest part. 



With respect to the sides of a stream, Col. Jacksou terms the heights at 



