DAVIS. OUTLINE OF CAPE COD. 321 



or retreat between the advancing bars and the retreating cliff. It 

 should not be overlooked that 'long-shore action has a share, often a 

 large share, in the development of compound forms of this kind ; but 

 it is quite conceivable that they might be developed essentially under 

 the control of on-and-off-shore action alone. A second example of 

 this kind is perhaps to be found in the combination of the bars from 

 Chatham to Nauset with the cliffed margin of the Cape mainland 

 farther north ; but into this problem it is not desirable to enter further 

 at present. The origin of tangent bars or spits, built out into compar- 

 atively deep water, may now be taken up. 



Tangent Bars or Spits. 



In order to understand more clearly the conditions under which 

 tangent bars would form, it is necessary to return for a few moments 

 to the problem of the varying outline of a graded shore as dependent 

 on an .increase of load. It is advisable to enter this phase of the 

 problem through comparison again with the development of rivers and 

 valleys. 



In the case of adolescent rivers, the increasing dissection of the 

 drainage basin by growing headwater branches may frequently cause 

 the load to continue to increase after the first attainment of a graded 

 slope along the trunk river. As a consequence, the trunk river must 

 aggrade the valley floor, forming a flood plain, until the load begins to 

 decrease later on in maturity. Much in the same way, 'long shore ac- 

 tion of the sea on a coast of graded outline may gather an increasing 

 load as the cliffs retreat and become longer and higher ; and with this 

 increase of load, certain parts of an early-graded outline may have to 

 be built forward into the sea. But on pursuing this comparison a step 

 further we find here, as in some earlier cases, a contrast replacing the 

 agreement thus far traced between the river and the 'long-shore action. 

 Not only the load, but also the volume of a river increases from youth 

 to maturity by reason of the better development of stream lines all 

 over the drainage basin ; and this increase of volume tends to prevent 

 the aggradation asked for by the increase of load. Similarly, the vol- 

 ume of water involved in the 'long-shore movements becomes greater 

 as the inequalities of a young shore line are reduced to the smooth 

 curves of adolescence and maturity ; but here the increase of volume 

 causes the shore waters to move in curves of larger radius than before, 

 and this change may require the beaches to grow forward on certain 

 concave or incurved parts of the shore line. In such case, increase in 

 the volume of 'long-shore water movements may co-operate with the 



VOL. XXXI. (k. s. xxiii.) 21 



