Vol. 67.] ANNIVERSARY ADDRESS OF THE r RESIDENT. lxvii 



(a) 'Thalassic Period.' 



(1) So long as deeper- water (' thalassic ') deposition is taking 

 place, the extent and depth of water is sufficient to allow of the 

 wide dispersal of sediment before it finally reaches and subsides 

 upon the sea-floor, and hence the areas of similar sediment 

 deposited will be wide. This is abundantly proved by soundings. 

 In these circumstances each lamina in such a deposit must answer 

 to a single sea - floor, and be approximately contemporaneous 

 throughout its entire horizontal extent. These lamina? will, 

 therefore, correspond with time-planes. Their outer margins will, 

 however, grade into organic, and their inner margins into coarse 

 sedimentary, deposits. In the deposits of the outer margins 

 correlation by organisms, if carried progressively from point to 

 point, ought not to present very serious difficulties. 



Whether any evidences of still more profound ocean-depths, 

 in the form of oceanic or abysmal deposits, occur among British 

 strata has been a subject of considerable discussion. For a reason 

 to be stated later, such deposits are not to be expected on an 

 extensive scale in Britain. It has been maintained that some of 

 the radiolarian cherts are not truly abysmal deposits, but it is 

 noteworthy that these rocks are practically free from terrigenous 

 material. 



(b) 'Deltaic Period.' 



(2) As uplift proceeds it not only elevates the land but brings 

 the floor of the area of deposition nearer to the surface of the sea. 

 Denudation becomes more active, and the amount of coarse detritus 

 carried out and deposited in shallow water increases in quantity. 

 Serious mistakes in correlation must inevitably result if the ' strata ' 

 thus laid down are interpreted in the same way as in a deeper-water 

 deposit. We know of no means by which sand can be spread out 

 under water simultaneously over large areas, so as to form a sheet 

 contemporaneous throughout. Even during the depression of the 

 estuarine phase, while the area is expanding, the deposit will 

 grow landwards and gradually envelop the submerged land-form ; 

 thus the inner or landward parts will be the newer. During 

 uplift, again, when the area of deposition is contracting, two things 

 will occur : — (i) The deposit will travel outwards and seawards 

 from its margin, and the outer parts will be the newest; (ii) those 

 portions of the deposit nearest land, and therefore the first to 

 feel the uplift, will be subject to denudation, and will be partly 

 broken up and redistributed as a new deposit farther out to sea. 



