EXAMPLES OF TRANSGRESSIVE OVERLAP 729 



stones are older continental deposits slightly reworked by an ad- 

 vancing sea. This appears also to be the case in the Cambric of 

 North America. The basal Cambric sandstones and conglomerates 

 of the southern ^Appalachian region underlie the Olenellus-bearing 

 shales and limestones, while those of the Oklahoma and Ozark 

 regions underlie beds generally referred to the Aliddle Cambric. 

 Finally, in the upper Mississippi Valley the St. Croix sandstone 

 series actually contains in its upper portion the Cambro-Ordovicic 

 transition fauna. In many cases this northern "Potsdam"' sand- 

 stone shows evidence of continental origin in pre-marine time by 

 the occurrence of well-marked torrential cross-bedding in parts 

 which apparently have not been reworked. In the North American 

 Cambric there are numerous distinct breaks, the magnitude of 

 which is not yet fully ascertained, except that it is now generally 

 recognized that above the Middle Cambric there is a hiatus corre- 

 sponding to nearly the whole of the Copper Cambric of the Atlantic 

 coast. These breaks are, as a rule, not marked by retreatal inter- 

 calated sandstones. Further examples of overlap involving large 

 portions of a system are shown in the North American Palaeozoic 

 by the entire absence of the Lower Cambric at St. John, New 

 Brunswick, where the basal marine elastics belong to the Middle 

 Cambric, while only 30 miles northeastward, at Hanford Brook, 

 the Lower Cambric (Etcheminian) has a thickness of 1,200 feet. 

 In Cape Breton this thickness measures several thousand feet. The 

 well-known fact that the Cambric of Bohemia begins with the 

 Paradoxides beds of Middle Cambric age, while Lower Cambric 

 beds occur in western Europe, shows a pronounced eastward trans- 

 gression of the Cambric sea in Europe with corresponding overlap 

 of formations. 



The basal Mesozoic sandstone of the Texas and Mexico re- 

 gions furnishes another typical example of a rising basal bed in a 

 transgressive series. In central Mexico this basal sandstone lies 

 beneath L'pper Jurassic limestones ; on the Tropic of Cancer it 

 has risen into the base of the Comanchic; on the Texas-Oklahoma 

 line it has risen through the Lower Comanchic (Trinity) and lies 

 at the base of the Fredericksburg or Middle Comanchic ; and, 

 finally, in central Kansas it has passed up to near the base of the 

 LTpper Comanchic or Washita series. There are, however, one pro- 

 nounced (Paluxy) and several smaller sandstone members inter- 

 calated in the limestone series, and these mark either shoaling or 

 an actual emergence of the sea-bottom. 



In this case, as in the Cambric, the basal sands are most prob- 

 ably of continental origin reworked by the transgressing Coman- 



