188 THEOKYr 



of the same height on both sides of the river. This process might be conceived to take place 

 at different levels, and produce all the lateral terraces. This is a view suggested by 

 Robert Chambers, and is adopted by many geologists. It is probable that many terraces 

 were formed in this way, but it will be impossible to suppose all thus produced, as the 

 number ought to correspond generally upon both sides, and their tops should always be 

 at the same height upon both sides which is not usually the case. Besides, the amount 

 of detritus which would in this way be carried into the principal streams by their trib- 

 utaries, or into the ocean, would be much greater than what is actually transported. 



2. Instead of supposing the whole valley filled with detritus, let us imagine that a bank 

 only is formed, fringing the valley below the water ; either by the materials of the deltas 

 of tributaries transported by the principal current and lodged near the shores; by the 

 accumulation of sand, gravel etc., brought into the water on the shores, by the number- 

 less little rivulets and streams that flow from springs perennially, or are brought into 

 existence by every rain-storm ; or by both these agencies united. If the level of the water 

 continues to be the same for a long time, as when a barrier exists at the end of the basin, 

 a lateral bank or terrace will form below the surface, which will be terraces, one on each 

 side, whenever the barrier is removed. In this case also the terraces will be of the same 

 height upon both sides of the valley, and not slope with the current, and there may be 

 several terraces upon each side in this case, provided only that the barrier shall exist at 

 several successive elevations. 



Several terraces in Vermont may be referred to this origin. There arc two kinds of 

 them, those formed by the sudden bursting of the barrier, and those formed by the grad- 

 ual removal of the barrier. Of the former class are the lateral terraces formed upon 

 Runaway Pond in Glover, upon Lime Pond in Williamstown, and wherever any pond 

 has suddenly left its bed in this way. Of the latter kind are probably the upper terraces 

 at Proctorsville upon Black River. We do not think that any basin of terraces has been 

 entirely formed in this way, because the terraces are of uneven height and have a lateral 

 slope, the latter of which could never have been formed in a quiet pond or lake. 



Another way in which lateral terraces are now forming, seems to have been the 

 most common method of their production. It is thus described by Dr. Hitchcock. 

 "Powerful freshets occur not unfrequently on all rivers ; and in their swollen condition, 

 and with increased velocity, they act powerfully upon their banks, especially if of alluvial 

 materials. And if the course of the stream be tortuous, as is always the case, one bank 

 will be acted upon more powerfully than the other. This action will produce a meadow 

 on one side of the stream, but little raised, it may be, above the river in its ordinary state. 

 Successive inundations will eat away the bank more and more, and thus widen the 

 alluvial flat. The stream will thus be spread out over a wide surface during its floods, 

 and of course its velocity will be lessened. This will cause a deposition of suspended 

 matter to take place, whereby the meadows will increase in height. Meanwhile the 

 stream will continue to wear its channel deeper, the supposition being that the drainage 

 is still going on. At length the channel will become so deep, and the meadows so high, 

 that even in freshets the waters will not spread over the meadows. They have now 

 become a permanent terrace, bounded by the river on one side, and by a steep escarp- 

 ment on the other, that leads to the higher terrace. 



