GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 247 



submarine flats separated by relatively steep slopes or escarpments, that 

 are due either to marginal wave cutting by the sea or are due to the 

 formation of a subaqueous profile above a previous profile;^ (c) the 

 presence, especially in limestones, below sea level, of solution wells, 

 pits, and caverns that inferentially were formed subaerially by the 

 solvent action of fresh water; (fl) the presence inland of free openings 

 that connect with the sea, showing that there are underground 

 channels by which ground water formerly flowed to the sea; (e) the 

 presence of submerged peat bogs or swamp deposits composed of 

 plants that grow only at or above sea level; (/) the presence below 

 sea level of indurated limestone, the induration of which is due to 

 solution of some of the original material and subsequent redeposi- 

 tion;2 {g) erosion unconformities at the bases of marine formations, 

 showing that there was subaerial erosion of the basement previous 

 to the submergence during which the formation was deposited or 

 accumulated in the sea. 



The foregoing statements might be elaborated, but to do so seems 

 unnecessary. The criteria enumerated are those I have actually 

 used in my own work. 



Besides ascertaining the proper succession of changes in the posi- 

 tion of strand hne, it is essential that the amount of the oscillations 

 be measured, that differential crustal movements be noted and dated, 

 and that an estimate be made of the endurance of the strand line in 

 its relation to present sea level. 



CRITERIA FOR MEASURING THE AMOUNT OF VERTICAL SHIFT IN STRAND LINE, 

 AND FOR DETERMINING THE RELATIVE AGES OF TERRACES AND THE PHYSIO- 

 GRAPHIC STAGE ATTAINED BY A SHORE LINE. 



The criteria for estimating the exact amomifc of rise or fall of sea level 

 are not yet definite, because adequate study has not been made of 

 the factors that determine effective wave base and of the depth to 

 which effective wave cutting extends. Notwithstanding this inade- 

 quacy of precise information, an approximation of the amount of 

 change may be made. In the case of elevation, the base of a wave- 

 cut escarpment or cliff, the flats of marine terraces, and wave-cut 

 grooves on sea cliffs, may be assumed to represent approximately 

 former sea level. Approximate measures of the amount of sub- 

 sidence may be based upon the depth of drowned valleys, the depth 

 below sea level of the bottoms of submerged solution wells and 



1 For discussions of this subject see as follows: 



Barrel!, Joseph, Factors in movement of the strand line, Washington Acad. Sci. Joum., vol. 12, pp. 413- 

 420, 1915; Factors in movements of the strand line and their results in the Pleistocene and Post-Pleisto- 

 cene, Amer. Joum. Sci., ser. 4, vol. 40, pp. 1-22, 1915. 



Vaughan, T. W., Some littoral and sublittoral physiographic features of the Virgin and northern Lee- 

 ward Islands and their bearing on the coral reef problem, Washington Acad. Sci. Joum., vol. 6, pp 

 53-66, 1916. 



2 The fundamental principle of this criterion is discussed on p. 250, under the caption "Solubility of cal- 

 cium carbonate in sea water." 



37149— 19— Bull. 103 5 



