268 Joseph Barrell. 



Marine conglomerates are "limited to considerably less 

 than 100 feet in thickness," while terrestrial ones are 

 "frequently measured in hundreds and occasionally in 

 thousands of feet." 



Having finished the paper on conglomerates, Barrell 

 now sought out a thick and unfossiliferous conglomerate- 

 sandstone series whose age relations were obscure. Such 

 a series he found in the southern Appalachians, and upon 

 this he made another study that remains Unpublished. 

 The conglomerates, sandstones, and slates of the Ocoee 

 and Chilhowie groups of Tennessee, North Carolina, and 

 Alabama, have long been a stumbling block in classifi- 

 cation, because it is only near the top of the long series 

 that fossils have been found. On the basis of these 

 fossils, Keith finally referred all of the Ocoee and Chil- 

 howie to the Lower Cambrian. Barrell discusses the 

 many formations of this series, aggregating between 

 9,000 and 13,000 feet in depth, and concludes that the 

 lower part, with a maximum thickness of 7,500 feet, is 

 of terrestrial origin. The middle formations, with a 

 thickness of 3,400 feet, are at least in part a terrestrial 

 deposit, though a part is probably marine. The remain- 

 ing 2,700 feet are entirely of marine origin. 



In 1912, Barrell, in continuation of his studies of 

 sedimentary formations, published the paper entitled 

 "Criteria for the Recognition of Ancient Delta De- 

 posits." He defines a delta "as a deposit partly sub- 

 aerial built by a river into or against a body of 

 permanent water." This study concerns the detailed 

 structures of deltas and the physiography of the land 

 that furnished the detritus. It is a difficult study be- 

 cause of the great variability in the extent of deltas, the 

 marked variation in the character of their sediments, 

 the size and streaming power of the river or rivers that 

 bring the material, and finally the wide variation in the 

 wave and streaming forces of the water body and the 

 depth of the water in which and in front of which the 

 deltas are laid down. 



The underlying principle of this work is the cyclic 

 one. The erosion by rivers "passes through its cycle 

 of youth, maturity, and age, and the characteristics of 

 the river valley and river waste change with the dis- 

 tance from the headwaters and with the progress of the 

 erosion cycle. There must also be a delta cycle, and it 



