10 Schuchert and Barrell — Revised Geologic 



areas of erosion and subtraction, it is all the more astonishing 

 to learn of the great thicknesses of some of these accumula- 

 tions. Such, however, were made in regions w T hich were under- 

 going pronounced subsidence. In Connecticut the coarse red 

 Triassic formations attain to a maximum thickness of 13,000 

 feet, and in New Jersey to upward of 20,000 feet, while in 

 Scotland the Old Red Sandstone of Devonian time also has a 

 thickness of 20,000 feet. These deposits are the remains of 

 beheaded mountains, the tops of wdiich are deposited in their 

 former valleys, that is, they are "intermontane continental 

 formations." Again, other thick continental formations are 

 intimately connected with marine deposits. Here we pass 

 almost insensibly from the ocean or sea across the brackish 

 water delta into the area of fresh-water deposition upon the 

 land. A good example is the great Appalachian delta of 

 Devonian times, the deposits of which in Pennsylvania reach a 

 maximum thickness of upward of 10,000 feet. Because of 

 these natural interfingering conditions, we are often able to 

 n't the marine record into that made on the land, and so use 

 both toward a more complete physical and organic chronology. 



The evidence of erosion. — Geologic chronology has been so 

 far almost wholly, though necessarily, interpreted on the basis 

 of stratified rock accumulations, that is, the marine and con- 

 tinental strata. There is, however, still another record that 

 has so far been almost refused recognition in our time-tables. 

 This is the time evaluation of topographic form at any given 

 stage of development (the physiography of the present, the 

 paleophysiography of the past). To be sure, it is mainly a 

 condition of removal by erosion of previously made histories, 

 but nevertheless the topographic form of the land still remains 

 and has a time value. We all appreciate to a certain extent 

 the significance of unconformities as records of emergence and 

 erosion between periods of inundation, but can any one tell 

 w T hat time value is to be accorded to the complete removal to 

 sea-level of mountain ranges like the present Alps of southern 

 Europe ? Many times have similar mountain chains been 

 washed away and then rejuvenated to some extent, only to be 

 worn away again after each reelevation. 



The "breaks" and "lost intervals" are known to be many, 

 but they are far greater in number, and their time durations, 

 although admittedly very variable, are far longer than is 

 usually believed to be the case. The geologic column will 

 probably never be completed on the basis of the recoverable 

 physical and organic evidence, but it will grow into greater 

 perfection for a long time to come, and this growth will take 

 place through the discovery of formation after formation along 

 the lines of these breaks, and more particularly in the areas 



