﻿Vol. 6 I .] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxiii 



representing change of lithological character, and those which 

 mark change in the assemblages of organisms. Each type may 

 be marked by broken or by zigzag lines. A particular kind of 

 sediment may accumulate locally, while adjoining tracts are not 

 receiving sediment ; or an organism, or group of organisms, may 

 spread out from a place for a short distance around the point 

 of origin. These would give rise to discontinuous lines in the 

 geogram. Again, owing to changes in the position of the sea-coast, 

 the strip of coastal sediment may shift. If, for instance, a coast 

 facing a sea to the west of it is undergoing submergence, the 

 coastal sediments will be deposited more and more to the east, as 

 the land sinks, and, when submergence is replaced by emergence, 

 these sediments will be deposited more and more to the west, 

 reposing on a wedge of deeper- water sediments, which were formed 

 when the coast lay more to the eastward. In this way a Y-shaped 

 deposit of coastal sediments will be accumulated, with the apex of 

 the Y pointing eastward. A series of such oscillations will give 

 rise to a series of Y's, forming a zigzag line with the apices pointing 

 alternately eastward and westward. The same thing will, of course, 

 occur with the various deeper- water sediments. 



Similarly, migration of organisms may cause a dominant organism, 

 or group of organisms, to be distributed throughout the strata in 

 zigzag lines. These lines may sometimes coincide, in whole or in 

 part, with those marked by variation in the lithological characters 

 of the strata, or may run independently of the latter. 



But the lines of a wide geogram, unlike thos-e of a meteorogram, 

 will be blurred. The sediments will pass one into the other 

 laterally as well as vertically, and the assemblages of organisms 

 will not change suddenly as a whole, but more or less gradually. 



The planes of bedding will also appear irregular in a wide 

 geogram. Two planes, when traced laterally, will often come 

 together, and a plane which is well-marked in one place may 

 completely disappear elsewhere. 



Accordingly, though any lines which we observe will probably 

 appear parallel in a narrow geogram, they will vary in an irregular 

 manner in one of considerable width. Now, in the meteorogram we 

 have another set of lines. Yertical divisions are drawn through 

 the meteorogram to mark days, and minor ones to mark hours. 

 These are absent in the geogram, and it is the task of the strati- 

 graphical geologist to attempt, as far as possible, to draw lines 

 separating one synchronous strip of strata from another. The 



