Theory of Formation of Sedimentary Deposits. 115 



(including, for convenience in discussion, the coarser 

 varieties, breccias and conglomerates), at or near the shore 

 line. As distance from the shore line increases (tempora- 

 rily assuming a gradually deepening of the water seaward) 

 these arenacious materials will grade gradually through 

 areno-argillaceous to argillaceous, and thence through 

 argillo-caleareous to caleareous, as the distance from the 

 shoreline increases. In Figures 1, 2, and 3 (omitting 

 zones of transition) A represents the sinking oldland upon 

 which the sediments are being laid down and from which 

 the detritus is derived, B represents the arenaceous zone, 

 C the argillaceous, and D the caleareous, the relatively 

 narrower transition zones between B and C, and between 

 C and D, being conveniently represented by lines. During 

 the course of time these deposits become more or less 

 coherent, according to the conditions to which they may 

 be subjected. In the subsequent discussion these separate 

 zones of deposition may be referred to as the sandstone, 

 the shale or the limestone zone, it being, however, recog- 

 nised that there are also zones of transition between 

 adjacent pairs. 



It would seem that any given stratum must have three 

 synchronous members, each merging gradually into the 

 adjacent member or members. The beds composed of 

 strata, which have been deposited successively must also 

 each consist of these three members. The fact that the 

 thickness of the stratum must, in each of the zones, be 

 dependent upon the amount of material present must 

 necessarily lead to the recognition of the other fact that 

 under certain conditions of supply a stratum of considerable 

 thickness might be laid down in one zone, while the 

 strictly synchronous equivalent in either of the other 

 zones might be represented by a very small amount of 

 deposit, or by none at all. Through the operation of 

 certain imperfectly understood causes, probably climatic, 

 directly or indirectly, strata are found grouped together 

 into beds, and it is conceivable, though not always proo- 



