STRATlGRAPHiC TAXONOMY 617 



ciently known to decide as to which contains the oldest beds and the most 

 complete sequence of deposits. It seems certain, however, that Cambrian 

 sedimentation began earlier in parts of the Cordilleran basin than in the 

 Appalachian troughs, or in an}^ other region in which rocks of this 

 period are known. Fot details concerning the lower Cambrian deposits 

 in western North America the reader is referred to Walcott's description 

 in the paper cited. 



Lower Cambrian deposits in southeastern North America. — The lower 

 Cambrian depositional sequence in the Appalachian tract has been fairly 

 well worked out in three areas. Beginning in the north, the first of these 

 is in Washington County, New York, and in the vicinity of Georgia, 

 Vermont. The base of the section here is unknown. As worked out by 

 Walcott, the lowest exposed formation is a limestone about 1,000 feet 

 thick. This is succeeded by 200 feet of "Georgia shale," and over this 

 comes 3,500 feet of shale and thin limestone. A quartzite 50 feet thick 

 follows and is in turn overlain by 1,700 feet of limestone and shale, and 

 this by 3,500 feet of shale. I have seen only the upper part of this 

 section, namely, the heavy bed of shale last mentioned, which, together 

 with all the underlying beds, has been referred to the lower Cambrian by 

 Walcott. Eegarding this upper shale I am strongly inclined to view it aSu 

 of Canadian age rather than Cambrian. There is some question in my 

 mind, also, concerning the age of a limestone supposed to belong to the 

 1,700-foot bed. It contains small bivalved phyllopods (e. g., Indiana 

 dermatoides (Wale), /. pyriformis Matth., I. secunda M., Bradoria 

 scrutator M.), which are characteristic of lower Acadian zones in New 

 Brunswick and Newfoundland. As it is not yet decided whether the 

 Protolenus zone is late lower Cambrian or early middle Cambrian, the 

 exact age of the Vermont bed mentioned also remains uncertain. Be- 

 sides, the structural geology of Vermont and of the Taconic area in 

 general is very complicated, so that it is not unlikely that future in- 

 vestigations may bring about considerable modifications of the Eopale- 

 ozic sequence now accepted for that region. 



The lower Cambrian sequence, exposed in and along the foot of South 

 Mountain in Pennsylvania, Maryland, and northern Virginia, is prob- 

 ably more accurately determined than the Vermont section. Besides, 

 the section here exposes the base in contact with pre-Cambrian rocks. 

 Beginning with the arkosic Loudon formation, the section consists in the 

 order of deposition of (1) the Weverton sandstone, which in places may 

 include the horizon of the Loudon and then attains a thickness of about 

 1,250 feet; (2) the Harpers schist, with a maximum thickness of 2,750 



XLI— Bull. Gkol. Soc. Am., Vol. 22, 1910. 



