OVERLAP ON PENEPLAIN 731' 



tions, but also the progressive overlap of facies in the same direc- 

 tion. 



b. Rate of Depression Exceeds Rate of Supply. LTnder these 

 ■ conditions there will be a rapid transgression of the sea, and the 

 meager supply of detritus will be spread thinly over the sea-floor, 

 or, if the depression is a rapid one, in many places deeper water 

 deposits may accumulate upon the original old land surface. Such 

 cases are known and are probably less infrequent than one is led 

 to suppose from the scattered observations available in the litera- 

 ture. A\'ilson (8:148) cites the case of a calcareous conglomerate 

 of Black River age, carrying angular quartz fragments, molds of a 

 Cameroceras and fragments of crinoid stems, which rests directly 

 upon the Archrean red granite near Kingston Mills, Ontario. From 

 the basin of the ]\Ioose River, Devonic corals have been reported, 

 with their bases attached to an Archsan abyssolith. (Parks-6 -.iSS.) 

 As under normal conditions of transgression, with an equivalent 

 supply of detritus, the change in lithic character is a gradual one 

 from coarse at the base to fine at the top, so, in any rapid or sudden 

 transgression, we would expect to find an abrupt change from 

 coarse material to fine, or from near-shore to off-shore deposits. 

 Conversely, where we find a sudden change from coarse beds be- 

 low to fine beds above, we may postulate a sudden relative change, 

 either a sudden transgression or a sudden diminution in the supply 

 of material. Sometimes a sudden transgression will transfer the 

 shore zone, from which much of the detrital material is supplied, 

 from one lithic formation to another, when the character of the 

 deposit will change. Thus an abrupt change of sea-level may 

 transfer the shore from a broad outcropping belt of quartz sand- 

 stones to a parallel belt of limestones, the sandstones being covered 

 by the encroaching sea. As a result, the deposition of quartz sands 

 may cease, and fine calcareous muds derived from the erosion of 

 limestones may be deposited upon the coarse quartz sands without 

 transition. Such a change appears to have taken place in eastern 

 North America in late Siluric time, effecting a change from the 

 Salina silicarenytes (Binnewater sandstones) to the fossiliferous 

 limestones and water-limes (Rosendale cement) which overly them. 



Where transgression takes place over an old peneplain surface, 

 on which residual soil has accumulated during the long period of 

 exposure, this ancient soil may be incorporated, without much 

 change, as a basal bed. Where the soil is lutaceous, especially 

 where it is a residual clay containing much carbonized vegetable 

 material, as in the case of an old swamp-covered surface, a black 

 carbonaceous mud will constitute the basal layer, which is sue- 



