432 THE CAMBRIAN. [bull.8L 



The question is asked, is the term Cambrian to stop at the first 

 unconformity below the Potsdam sandstone ? Is it to extend to the 

 second of those u ncom form i ties, or to the third ? Or is it to include, 

 finally, the lowest of the formations of the region? 



He makes the following reply to these questions: 



All these usages of the term have been made. In taxonomical value the term Cam- 

 brian is designed, of course, to correspond to the terms Upper Silurian, Lower Silurian, 

 Devonian, Carboniferous, etc., which terms are based primarily upon the general con- 

 tinuity for each one of the periods to which they correspond of similar life conditions. 

 In no case, however, has one of these terms been allowed to span any such unconform- 

 able break as the least of those beneath the Fotsdam sandstoue of the Lake Superior 

 region. Minor breaks in the succession have been included within a single one of 

 these geological groups because — although in every case of a physical break, even the 

 smallest, a corresponding break in life-succession is found — there have been found on 

 both sides of the break fully developed faunas of general similarity in characters; 

 but no great break in the succession such as those with which we are now concerned 

 has ever yet been found to be spanned by a continuity of forms. The conclusion, 

 therefore, seems inevitable that we should not extend the term Cambrian over any 

 such break, at least until there shall have been found closely corresponding faunas 

 on opposite sides of the interval. Were such faunas to be discovered the greatness 

 of the unrecorded interval would still remain, and we should have indicated only a 

 .singularly long continuance of similar life-conditions. Even then the question might 

 arise as to whether continuity of life-conditions should outweigh the great lapse of 

 time indicated by the physical hiatus. 1 



Finally Dr. Irving states that 



The structural breaks called unconformities are properly used in classification — 



(1) To mark the boundaries of the rock groups of a given region. 



(2) To aid in establishing correlations between the formations of different parts of a sin- 

 gle geological basin. 



(3) To aid in the establishment of correlations between the groups of regions distantly 

 removed from one another; but caution is needed in attempting such correlations in 

 proportion as the distances between the regions compared grow greater. 



They are improperly ignored : 



(1) Wlien the evidence they offer as to separateness is allowed to be overborne by anything 

 but the most complete and weighty of paleontological evidence. 8 



V. MISCELLANEOUS. 



Homogeny. — A method of correlation that has frequently been used 

 has recently been named by Mr. W J McGee "Homogeny" or correla- 

 tion by community of genesis. 3 Mr. McGee defines it as follows : 



The method of correlation devised to systematize the structure of the Coastal Plain 

 combines the desirable features of the older methods, and adds thereto the interpre- 

 tation of the products of the several physical processes operating upon the earth's 

 exterior. 4 



The older methods mentioned are those of strati graphic continuity, 

 lithologic correlation or correlation by petrography , and paleontologic 



1 Op. cit, p. 446. 3 Am. .Tour, Sci., 3d ser., vol. 40, 1890, pp. 36-41. 



2 Op. cit., p. 448. « Op. cit., p. 36. 



