350 HATCHER MARINE AND NON-MARINE FORMATIONS. [April 7. 



extent assuming that conditions were more or less uniform over the 

 entire region occupied by the formation from the beginning to the 

 close of its deposition — an assumption wholly unwarranted, as we 

 have seen in formations deposited under conditions similar to those 

 represented in the first diagram and more especially in the littoral 

 and brackish water beds, represented respectively by 2 and 3. In 

 beds such as these the actual area over which deposition takes 

 place at any given time is limited to a narrow strip along the coast, 

 and materials are there being added simultaneously and continu- 

 ously throughout the entire vertical extent of both these beds. 

 Jt is thus evident that in deposits such as these geographical dis- 

 tribution rather than vertical thickness is the basis upon which to 

 estimate the length of time employed in their deposition. The 

 age of any fossil found in these beds will be greater or less than 

 that of another found at a different locality, not according as to 

 whether it occurred at a higher or lower level in the series, but 

 according to the difference in the geographical position of each 

 relative to the original coast line, and the actual difference in age 

 will, of course, depend upon the difference in the geographical 

 positions at which the two occurred and the rate of advance or 

 recession of the coast. The difference in age would be expressed 

 in years by dividing the distance between the two localities, meas- 

 ured along a line at right angles to the original coast line, by the 

 annual rate of advance or recession of the coast. 



Let us consider again the conditions at present existing along 

 our Atlantic coast and represented in the first diagram. It will be 

 readily apparent that as the coast line is advanced eastward by the 

 deposition of the material brought down by the streams, the western 

 limits of true marine, littoral and brackish water conditions will 

 be more and more restricted and that marine conditions with a 

 depth of sea of more than 2000 feet will prevail over the region 

 near the eastern border of the submerged continental plateau long 

 after the present coast line has advanced to the position shown at 

 X. on the diagram, while to the westward of this position the 

 region has all been brought above sea level and only fresh water 

 and aeolian deposits are being formed. It is evident then that 

 when the coast line has been advanced to a." the uppermost 

 stratum of the marine formation. No. i, will not be of uniform 

 age from X. to K, but that this stratum at X. will exceed in age 

 the same stratum at Y. by the length of the time required for the 



