ON THE COAST OF MAINE. 



oo — 



Fig. 3. 



In Fifj. 3, the pivot point, P, is to the landward of the shore, S, the 

 hne A B inilicating the surface of the continent near the coast. Let 

 the depression of tlie sea floor and the elevation of the land go on until 

 the continental surface is in the position indicated by the lines A' B', 

 and the shore will be removed to the point S', and the sea gains. S L, 

 indicates the sea level. If we suppose, however, that the dotted line P 

 L', denotes the sea level, then the pivot point will fall just at the shore 

 line, anil all the ch mges in the position of the line A B will not alfeet 

 the 2)osition of the water lines. 



succinctly stated, give us evidence of the truth of the natural supposition that concurrent 

 with the rise of the land there has been a constant increase in the depth of the sea. 

 If we try, however, to reduce all 



,1 i> 1 1 i ii i In the diagrams 3 and 4, similar letters denote correspondin<r points. 



ffreat chansres oi level to the two , ^ „ , , », ,■■,,,■ a r. w r./ ,• 



<= ° _ In figures 3 and 4, the straight lines A B, A B , are diagrammatic 



movements, sinking of the sea and exjiresslons for sections e.\tending across the shore. For convenience of 



risiu"" of the land we are met by delineation, the action of the movement of the small segments of the 



,^ T m 1. 1 • 1 -n i •. crust represented is supposed to be like that of a ri<;id bar. 



the tlilnculty, which will suggest it- 

 self to all geologists, that the sea 

 and land have in many regions 

 changed places in alternation. It 

 seems to me, however, that this dif- 

 ficulty is in a great measure over- 

 come by the careful study of the 

 necessary conditions of the move- 

 ment. In diagrams Nos. 3, 4 and 5, 

 Ave have represented the line from 

 the centre of any sea to the interior 

 of any continental area. Assuming 

 that the constant tendency of the 

 movements is to depress the sea- 

 ward part of this section and lift the 

 landward part, we have to notice 

 that there must be a fulcrum point, 

 or point of rotation of the move- 

 ment. This point may occupy either 

 of three relations in reference to 

 the sea. It may be to the seaward 

 of the shore ; it may fall at the 

 shore line, or it may lie to the land- 

 ward of the shore. Supposing the 

 sea floor to be constantly sinking, 

 and the land constantly rising, it is 

 evident that in the first of these 

 suppositions the land would contin- 

 ually gain on the sea ; in the second 

 case, great changes might take place 

 without any effect on the po.sition of 

 the shore ; while in the third case 

 the sea would seem to gain on the 

 land. The fact that in most of the 

 land areas where we knew the geol- 

 ogy well enough to form an opinion 

 on the matter, there are centres of 

 ancient upheaval about which the 

 oscillations of level have taken place, 



MEMOIRS EOST. SOC. NAT. HIST. VOL. II. 



Fig. 4. 



In Fig. 4, the pivot point is the seaward of the shore line A B, indi- 

 cating the origin il position of the continental surface, and A' B', the 

 position of the change. Inspection will show that in this diagram the 

 change has caused the shore to move seaward, and the land gains. 



Fig. 5. 



Fig. 5 represents a shore line with an axis of rotation, A B, cutting it 

 in such a manner that the points P and \" may be taken for the pivot 

 points of the diagrams 3 and 4, respectively. All that portion of the 

 shore line above the straight line would be sinking; all below, rising. 



85 



