Appalachian Geosyncline. 107 



between variations in original thickness and the locations of 

 individual folds. The data show a general thickening from 

 the northeast end of the anthracite synclinorium toward the 

 Susquehanna River and, more rapidly, from the northwest 

 toward the southeast. There are certain irregularities in the 

 variations, some of them real, some of them due to the imper- 

 fections in measurement. On p. 257 and in PL XLVTII of 

 the article cited,* for example, the thickness in column 1 would 

 be about 1400 feet greater if the measurements of IT. M. Chance 

 had been used instead of those by Sherwood. Further, the dia- 

 grams of Pis. LXVIII and LXIX can be interpreted as show- 

 ing no relationships between initial dips and the individual 

 folds, an interpretation opposite to that which is given. 



As another angle of approach to the present topic of rela- 

 tionship between structure and original sedimentary limits, 

 the Appalachians may be compared to the larger plans of the 

 Tertiary mountain chains of Eurasia. From them Suess has 

 shown that in the typical mountain range the folds are over- 

 turned and overthrust in a convex arc upon a foreland plain. 

 The complexly folded and uplifted zone is an anticlinorium. 

 If this foreland becomes crumpled and depressed it constitutes 

 a synclinorium, — to use these terms as defined by Yan ITise. 

 The synclinorium tends to lie in front of the anticlinorium and 

 to dip beneath it. The overthrust acts through the uplifted 

 mass, not from the ocean. The depressed mass is underthrust, 

 the two tending to slide past each other. The Juras and the 

 Great Valley of Switzerland with its ranges of Molasse hold 

 such a relation to the Alps; the sub-Himalayas to the Hima- 

 layas. In both cases these synclinoria contain upper strata 

 made from the debris of the first foldings of the anticlinoria 

 and have been added to the mountain system by a secondary 

 period of folding. The mountains have grown by adding the 

 marginal foreland. 



After profound planation across the system the anticlinorium 

 will show older and originally deeper rock formations, the 

 synclinorium those which were younger. To what extent 

 then may we argue that these younger formations never existed 

 upon the anticlinorium and that the steep or overturned limb 

 between them is the original boundary of the sediments ? The 

 answer is best given by the study of the Tertiary mountain 

 systems, since evidence of the original distribution of the strata 

 is in many examples not yet wholly destroyed. The Alps and 

 other Eurasian ranges show involved in their higher ranges 

 great thicknesses of Mesozoic rocks. Upon erosion to near the 

 baselevel these would doubtless be in large measure destroyed 

 *U. S. Geol. Surv., 13th Ann. Report, Pt. II. 



