1 138 PRINCIPLES OF STRATIGRAPHY 



In the same way the beds resting upon the truncated Ordovicic 

 folds in New York and Pennsylvania vary considerably in age. 

 The conglomerates succeeding the unconforniity were originally all 

 classed as basal Siluric. In reality some are of Lower and some 

 are of Middle Siluric age, while in still other cases beds of upper 

 Siluric or even younger age rest directly upon the truncated roots 

 of the old folds. 



Nevertheless, with due circumspection it is possible to use 

 great and widespread unconformities for broad correlation of 

 formations. Since there were two periods of widespread, if not uni- 

 versal, disturbance of the earth's crust, besides many minor ones, 

 these at least have a considerable value in correlation. One of 

 these occurred before the beginning of Palaeozoic time, and the 

 other came to an end before the beginning of Mesozoic time. Thus 

 everywhere, the world over, the Archaean rocks are separated from 

 the Palaeozoic formations by great unconformities. This does not 

 apparently hold for all pre-Cambric rocks, however, since some of 

 the formations commonly referred to the Algonkian are separated 

 from the Cambric only by a disconformity. It may, however, be 

 true, as already pointed out, that the unaltered or but slightly 

 altered rocks, like the Belt terrane, the L^inta sandstone, and others, 

 are not necessarily pre-Palseozoic. They are known to be pre- 

 Cambric, or better, in most cases only pre-Middle Cambric, and, 

 since they are largely, if not entirely, of non-marine origin, they 

 may, in part at least, represent the continental equivalents of the 

 marine Lower Cambric beds. The unconformity between the Palae- 

 ozoic and Mesozoic, though widespread, is nevertheless much more 

 restricted than the earlier one mentioned. Moreover, it is of suffi- 

 cient constancy to make possible this broad correlation, though, as 

 before remarked, there is no guarantee that the beds next succeed- 

 ing are everywhere of the same age. 



Disconformities are, to a certain extent, better criteria for cor- 

 relation, especially the larger and more extensive ones, which can 

 be interpreted as due to eustatic movements of the sea. Withdraw- 

 als or transgressions of the sea, due to change in sea-level, will 

 affect all continents more or less in the same manner, and thus 

 serve as a primary basis for subdivision. The danger with this 

 method lies in the difificulty of distinguishing between the local and 

 the widespread character of the disconformity and the tendency 

 which it induces to multiply the number of breaks in the geologic 

 column by assuming that the minor breaks of one locality are 

 necessarily reproduced elsewhere. 



That there are widespread breaks in the geological column, 



