STRATIGRAPHIC CLASSIFICATION DIASTROPHIC CRITERIA 445 



foundland, but lateral extensions across local depressions in the land to 

 the east, permitting free communication with the Atlantic sea, are 

 thought to be more likely. 



Finally, a fifth trough, or rather set of troughs, is thought to be indi- 

 cated by the highly metamorphosed Paleozoics found to the east of the 

 lower Cambrian outcrops. Theoretically, none of these should be older 

 than middle Cambrian, but many later ages up to the Jurassic might 

 be represented. Most probably, however, the sequence was frequently 

 interrupted. Local masses of sparingly fossiliferous slates and shales 

 in southwestern Vermont and eastern New York north of Albany do 

 not fit in with any of the deposits above assigned to the several troughs. 

 Dale^^ referred some of these slates to the Cambrian, but a recent study 

 of the fossils shows them to be of some Canadian age. Where I have 

 seen them in Vermont (especially near Sudbury) they rest unconform- 

 ably on the eroded folds of the marble, limestone, and shale sequence 

 characterizing the third trough. 



If, as I believe, these unassigned slates and shales really belong to 

 the deposits of the fifth trough, it will be necessary to devise some 

 plausible explanation of how they came into their present position. 

 Perhaps the following may be so considered: The shales of the eastern 

 belts were first folded and thrust westward by "suboceanic spread." The 

 latter, continuing and affecting deeper zones, then folded and pushed 

 the deposits in the lower Cambrian (fourth) trough over the edge of 

 the Eutland marble basin, the more ^^competent" Cambrian beds carrying 

 with them their previously acquired load of slates of the fifth trough. 

 With further folding and thrusting and removal of the advance edge of 

 the overthrusting lower Cambrian by erosion, it is readily conceivable 

 how the load of slate riding in a syncline might finally get into a position 

 favoring a gravitational slide into the limestone valleys of the third 

 trough. Indeed, in areas of less erosion and hence apparently greater 

 overthrusting, as in Washington county, and probably farther south 

 in New. York, slates of the fifth trough might have come into contact 

 with the shales of the Levis channel. 



If these supposed great overthrusts are due chiefly to suboceanic 

 spread, and if the idea of inland migration of the zone of active folding 

 is correct, then we have in the inland riding of the relatively shallow 

 older shear planes on the deeper inclined planes of the newer thrusts*^ 



*« T. Nelson Dale : The slate belt of eastern New York and western Vermont. U. S. 

 Geological Survey, 19th Ann. Rept, Part III, 1899, pp. 153-300. 



*'' In speaking of the superficial older sheets as riding on the inland moving new plane 

 I do not mean to say that the emerged parts of the former are no longer subject to inde- 

 pendent movement. On the contrary, I believe that the older sheets not only lagged 



