890 E. O. TILRTCH REVISION OF THE PALEOZOIC SYSTEMS 



of these formations, which are found in most instances still well up on 

 the flanks of the old lands, commonly do not include a depositional 

 record of certain earlier stages of the several transgressions that are well 

 represented in the thicker sections of central and southern Pennsylvania. 

 Indeed, some of the invasions from the south failed entirely to reach 

 the probable line of outcrop in New York. An instance of such failure 

 is the post-Lowville part of the Chambersburg limestone, a composite 

 formation that is rather fully described on pages 321 to 329. As defined 

 and mapped by Stose, the Chambersburg comprises three well distin- 

 guished members, all of them older than the base of the Trenton. The 

 middle division seems unquestionably a representative of the Lowville, 

 and following it there is an abrupt and complete change in faunas. 

 The upper member attains a maximum thickness of over 600 feet and 

 has every attribute of a distinct formation, with a time value quite equal 

 to that of the Trenton. Perhaps small parts of its time are represented 

 in the New York section by the recently named Watertown and Amster- 

 dam limestones, but considered as a stratigraphic unit, this important 

 upper member of the Chambersburg certainly failed to reach New York. 

 In fact, the head of the narrow bay in which it was deposited did not 

 extend north of Harrisburg, Pennsylvania. 



We have already discussed the question, how much reduction is per- 

 missible in an overlapping formation before another designation is 

 desirable? The preceding paragraph presents the opposite condition 

 of expansion. In my opinion, the only practical solution of this phase 

 of the question is to continue adding to the bottom of the stratigraphic 

 unit, be it of the rank of a member, a formation, or a group, until we 

 encounter beds clearly referable to the next underlying equal unit of 

 the standard section or some other well defined boundary. In the latter 

 event the expanded section may contain a new formation or other strati- 

 graphic unit that henceforth becomes an integral part of the geological 

 scale. 



BROAD INTERPRETATION RECOMMENDED FOR OLD NEW YORK TERMS 



As a rule, it seems desirable to give as broad an interpretation as 

 possible to the formation names finally adopted by the geologists of the 

 first New York Survey. They were used so widely by the older geologists, 

 and withal so loosely, even in New York, that their original significance 

 has in some cases become obsolete. It is from these chiefly that the 

 stage or group and epoch or series names should be selected. In their 

 revision of the New York section Clarke and Schuchert, in 1899 and 

 1903, the latter a slightly amended edition published by Clarke alone, 



