284 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



behavior of a meandering and swinging stream, slowly degrading a pre- 

 viously aggraded valley without change in volume ; and, second, by the 

 control exerted here and there over the lateral swinging of the stream 

 through the discovery of rock ledges, as suggested by Miller. The 

 following pages are devoted to a fuller consideration of this conclusion. 



II. Preliminary Inquiry. 



Various Kinds of Terraces. For the sake of clearness it is desirable 

 to exclude at the outset all kinds of terraces other than those here 

 studied. The terraces that occupy so many of our valleys are known as 

 river terraces, drift terraces, or alluvial terraces. They have as to 

 origin nothing in common with the terraces of sea-shores, such as occur 

 on the coastal slopes of Cuba ; or with the lake-shore terraces so well 

 developed in the basins of Bonneville and Lahontan. The} T bear little 

 resemblance to structural rock benches, such as break the slopes of 

 valley sides in dissected plateaus, as in West Virginia or on a still 

 larger scale in the Colorado canyon. They have little likeness to the 

 silt and gravel-covered rock terraces formed when a graded river, 

 revived by uplift, cuts a new valley in its former valley floor, as along 

 the gorge of the Rhine on its way through the Schiefergebirge of 

 western Germany. 



Our New England drift terraces have a flat and nearly level upper 

 surface or plain, limited backwards by rising ground and forwards by 

 falling ground, and to that extent they resemble the terraces of all the 

 classes above mentioned ; but they have certain well-marked features of 

 their own. They are evidently the river-carved remnants of a body 

 of stratified clays, sands, or gravels that once occupied in larger volume 

 than to-day the rock-floored valleys of still earlier origin. Their upper 

 surface, the terrace plain or floor, slopes with the fall of the stream by 

 which their scarped face or front has been eroded; and in this they' 

 differ from sea and lake shore terraces and from structural rock benches, 

 none of which have any particular relation to the slope of neighboring 

 streams. They consist of unconsolidated, stratified drift ; if a ledge 

 appears in any part of a drift terrace it is manifestly an accidental 

 element, although as will be shown it may exert a controlling influence 

 on the pattern of the terrace front ; in this our drift terraces differ from 

 the structural rock benches of valley sides in dissected plateaus, and 

 from the rock terraces that represent the former valley floors of 

 revived rivers, both of which consist essentially of rock, even though the 



