CLARK: TRENTON LIMESTONE AT MARTINSBURG. 9 



the occurrence of these three zones in the same order are, I beheve, 

 sufficient to indicate the correctness of the correlation of the portions 

 of the sections occupied by these faunas, even though the zones of 

 the two localities are not of the same thickness. The lower 390 feet 

 of the section at Martinsburg are therefore probabl}^ the equivalent 

 of the total 346 feet of the combined Trenton Falls-Rathbone 

 Brook section, and the upper eighty-five feet of the section at Martins- 

 burg have no equivalent in the limestone of the more southern section, 

 but form a younger deposit. This is the view already advanced by 

 Raymond, but supported by less evidence. 



While the general correlation between these two sections is obvious, 

 detailed correlation is attended with difficulties, and probably should 

 not be attempted until the intervening region can be studied. While 

 Cryptoliihus tessellatus occupies a zone about forty feet thick in which 

 it is abundant over a stretch of more than a thousand miles from east 

 of Quebec down past Montreal, the Champlain Valley, Saratoga, 

 and the Mohawk Valley to Rathbone Brook, at Martinsburg it is 

 found in only a single layer, and that twenty to thirty feet higher in 

 the section than it is normally found. Its western migration was 

 evidently delayed, and finally stopped by something other than a 

 physical barrier, but just what it was is not evident. At Martinsburg 

 the lowest beds are characterized especially by three species of Triple- 

 cia. At Trenton Falls one of these species, T. cxtans, is quite common 

 in one layer, but that layer is about seventy-five feet above the base, 

 and above tlie range of Cryptolithus, and not below it. The meaning 

 of this interchange of position is likewise not yet understood. 



The occurrence of Triplecia in the lowest zone at Martinsburg is 

 of value in making a correlation with the Trenton of Ontario. In 

 the section at Ottawa and vicinity (Raymond, Guide book 3, 

 Excurs. 12th Internat. geol. cong. 1913) one finds at the base thirty- 

 five feet of limestone with T. extans, Phragmolites compressus, and 

 other fossils; in the middle a thick zone with numerous fossils in- 

 cluding an abundance of Prasopora simulatrix. Then come two 

 zones characterized by Rafinesquina deltoidea and Strophomena tri- 

 lobata respectively, the general sequence being that at Martinsbiu-g 

 but with a different development of the zone between that of Triplecia 

 and that of Prasopora. 



These notes should suffice to show the great importance of the 

 Martinsburg section, since by combining in itself elements of the 

 faunas of the Trenton to the north and the Trenton to the south, it 

 permits a correlation which has formerly been in doubt. Perhaps 



