47t! O. Schuchei't — Pre-Cambrian Nomcnclatiire. 



another which has come into general use. In the latter case 

 the first name should lie dropped for this formation ; and, 

 where there can be no question as to the general usage of the 

 subsequent name, the first name might be used again for a dif- 

 ferent formation without causing serious trouble." " The 

 duplication of formation names has become a serious matter, 

 as will be seen from an examination of this list. . . . New 

 names should replace those which can not hold their present 

 varied definitions." 



Weeks has a separate list of the geographic names that have 

 been applied to masses of igneous rocks occurring in North 

 America. Two identical names, one for a stratigraphic unit 

 and another for an igneous mass, may therefore remain in use. 



The rule of priority as to formation, series, and period terms 

 is now generally adhered to, but in regard to era names the 

 rule is not so rigidly followed, because they are more expres- 

 sive of ideal conditions than are the smaller divisions of time. 

 It is the same in biology in relation to family, order, and class 

 terms, and it is from this source that paleontologists and geolo- 

 gists get their ideas of fixity in nomenclature. 



The principles of correlation adopted by Lawson (and to 

 which the writer also adheres) are. for pre-Cambrian forma- 

 tions : 



"(1) The principle of lithologic similarity and the com- 

 munity of conditions of deposition inferred from this similarity. 

 (2) The principle of the similarity of sequence. (3) The prin- 

 ciple of coincidence of unconformities in the sequence. (4) The 

 principle of irruptive contacts" (4). On the basis of these 

 principles the hypothesis of two periods of granitic invasion 

 "states that in post-Keewatin time there were two and only 

 two periods in which great granitic batholiths were developed 

 in the earth's crust in the region of the present Great Lakes" (5). 

 Furthermore, " in every one of the fifteen districts, considered 

 individually, geological time is blocked out into three grand 

 divisions by the two granite invasions : the pre-granitic, the 

 inter-granitic and the post-granitic " (12). 



In regard to the major unconformities Lawson correctly 

 holds that " Wherever the earth's crust is known to have been 

 extensively invaded by granite, an important concomitant con- 

 dition has been the uplift of the region affected and the inau- 

 guration of a prolonged period of degradation, culminating in 

 the removal of the cover from extensive areas of the gran- 

 ite. . . . The time necessary for the invasion of a region bj' 

 granite is unknown, but it may well have been a long drawn- 

 out process. The stripping of the cover of the granite, how- 

 ever, and particularly the reduction of a high region to low 

 relief, requires a long time in the geological sense ; and the 



