RIVERS. 283 



which they flow. Those of primary or transition countries, where sudden declivities 

 abound, are bold and rapid streams, with steep and high banks, and usually pure waters, 

 owing to the surface not being readily abraded, generally emptying themselves by a 

 single mouth which is deep and unobstructed* The streams of secondary and alluvial 

 districts flow with slow but powerful current, between low and gradually descending banks, 

 which, being composed of soft rocks or alluvial grounds, are easily worn away by the 

 waters, and hence great changes are effected in thoir channels, and a peculiar colour is 

 given to their streams by the earthy particles with which they are charged. Many rivers 

 have their names from this last circumstance. The Rio Negro, or Black River, which 

 flows into the Amazon, is so called on account of the dark colour of its waters, which are 

 of an amber hue wherever it is shallow, and dark brown wherever the depth is great. 

 The names of the two great streams which unite to form the Nile, the Bahr-el-Abiad, or 

 White River, from the Mountains of the Moon, and the Bahr-el-Azrek, or Blue River, 

 from Abyssinia, refer to the colour which they receive from the quantity of earth with 

 which they are impregnated. The united rivers, for some distance after their junction, 

 preserve their colours distinct. This is the case likewise with the Rhine and 1 the Moselle, 

 the St. Lawrence and the Ottawa. The Upper Mississippi is a transparent stream, but 

 assumes the colour of the Missouri upon joining that river, the mud of which is as copious 

 as the water can hold in suspension, and of a white soapy hue. The Ohio brings into it 

 a flood of a greenish colour. The bright and dark red waters of the Arkansas and Red 

 River afterwards diminish the whiteness derived from the Missouri, and the volume of the 

 Lower Mississippi bears along a tribute of vegetable soil, collected from the most distant 

 quarters, and of the most various kind, the marl of the Rocky and the clay of the Black 

 Mountains the earth of the Alleghanies and the red-loam washed from the hills at 

 the sources of the Arkansas and the Red River. Mr. Lyell states that water flowing at 

 the rate of three inches per second will tear up fine clay ; six inches per second, fine sand ; 

 twelve inches per second, fine gravel; and three feet per second, stones of the size of an 

 egg. He remarks, likewise, that the rapidity at the bottom of a stream is every where less 

 than in any part above it, and is greatest at the surface ; and that in the middle of the 

 stream the particles at the top move swifter than those at the sides. The ease with 

 which running water bears along large quantities of sand, gravel, and pebbles, ceases to 

 surprise, when we consider that the specific gravity of rocks in water is much less than 

 in air. .. : :j v 



It is chiefly in primary and transition countries that the rivers exhibit those sudden 

 descents, which pass under the general denomination of falls, and form either cataracts 

 or rapids. They occur in secondary regions, but more rarely, and the descent is of a 

 more gentle description. The falls are generally found in the passage of streams from 

 the primitive to the other formations. Thus the line which divides the primitive and 

 alluvial formations on the coast of the United States, is marked by the falls or rapids of 

 its rivers, while none occur in the alluvion below. Cataracts are formed by the descent 

 of a river over a precipice which is perpendicular, or nearly so, and depend, for their 

 sublimity, upon the height of the fall, and the magnitude of the stream. Rapids are pro- 

 duced by the occurrence of a steeply-inclined plane, over which the flood rushes with 

 great impetuosity, yet without being projected over a precipice. The great rivers of 

 England the Thames, Trent, and Severn exhibit no example of either cataract or 

 rapid, but pursue a generally even and noiseless course ; though near their sources, while 

 yet mere brooks and rivulets, most of our home streams present these features in a very 

 miniature manner. A true rapid occurs in the course of the Shannon, just above Lime- 

 rick, where the river, forty feet deep, and three hundred yards wide, pours its body of 

 water through and above a congregation of huge rocks and stones, extending nearly half 



