TRANSGRESSIVE OVERLAP 571 



ter in most of the sections. But it is evident from a consideration of the 

 mode of its formation that the age of different portions of this basal bed 

 varies, becoming progressively younger in the direction of transgression. 

 The following diagram will illustrate this principle: 



The series of successive strata, 1 to 7, is deposited at A during the 

 period of transgression of the sea from A to B, and therefore it consti- 

 tutes the depositional equivalent of the time interval occupied by the 

 transgression, which may be assumed to have proceeded at a uniform 

 rate. It is evident that the basal sand or conglomerate bed 1-8 is not 

 of the same age throughout, but rises in the scale progressively, until 

 at B it is equivalent in age to bed 7 at A. The same thing is true of 

 bed 2'-9, a finer bed which directly succeeds the basal bed, and which 

 like it rises in age in the direction of transgression. It is clear that two 

 sections of this series, taken the one nearer the shore than the other, as at 

 C and A, will have the same lithic succession from the base upward ; but 

 section C will begin very much higher in the scale than section A, and 

 the corresponding lithic units of the two sections will be of different age. 



Application of the Principle of Transgressive Overlap in the 



sedimentary series 



the basal paleozoic series 



General character of the overlap. — Wherever the Paleozoic rocks are 

 found to rest unconformably on' the pre-Cambrics, a comparison of sec- 

 tions shows a progressive overlapping of the successive formations, each 

 of which rests, with a basal sand or conglomerate bed, on the eroded 

 surface of the pre-Cambric old land. Some of the more typical exam- 

 ples of this may now be cited. 



Newfoundland. — A comparison of the following sections from Trinity 

 and Conception bays, Newfoundland, will show the character of the basal 

 transgression. At Trinity bay, Smith sound, the Lower Cambric (Etche- 

 minian of Matthew) is represented by 811 feet of fossiliferous shales, 

 with some limestones carrying the Holmia broggeri fauna. Almost 350 

 feet below the top of the Etcheminian is a brick red and pinkish lime- 

 stone stratum, 27 feet thick, and rich in Holmia broggeri, Hyolithes 

 princeps, and other fossils. This is the Smith Point limestone of Wal- 

 cott, which has been recognized in Conception, Saint Marys, and Placen- 

 tia bays. In Conception bay, at Manuels brook, this limestone when 

 found rests directly on the basal conglomerate, which has a thickness of 

 35 feet, and in its basal portion contains boulders of the underlying 

 gneiss up to 6 feet in diameter; but upward it changes to fine sand. 



