124 T. N. Dale — Alyonlcian- Cambrian Boundary. 



general N.NE. strike. Some pre-Cambrian rocks show the 

 effects of botli crustal movements. 



Whereverin the Green Mountain region both Cambrian and 

 pre-Cambrian have a N. 70°-90° W. strike it must be due 

 either to transverse folding in the course of the post-Ordovi- 

 cian movement, or, as surmised by Pumpelly, to compensatory 

 movement due to the resistance offered by rigid granite masses.** 



If the prevalent strike at the close of Algonkian time here 

 was WJW. then the original orographic features of the 

 Green Mountain region, or at least of the southern half of it, 

 must have trended W.NW.-E.SE., and therefore wherever a 

 ridge of pre-Cambrian rocks with this strike has this trend it 

 may be regarded as a remnant of Algonkian physiography. 

 The two mile long ridge in Andover and Weston, known as 

 Markbam Mountain, appears to be such an Algonkian moun- 

 tain-remnant. 



As, along the boundary studied, Cambrian rocks are in some 

 places in contact with Algonkian ones but in others with vari- 

 ous granite-gneisses, we must suppose in the latter places either : 

 (1) Denudation in Algonkian time of the Algonkian land sur- 

 face and the removal of the Algonkian sediments that had 

 transgressed the pre- Algonkian gneisses, or else (2) the expo- 

 sure of part of the land surface of Algonkian time which was 

 transgressed by Cambrian sediments but never had been by 

 Algonkian ones. 



As the Algonkian conglomerate contains pebbles of quartz- 

 ite and conformably overlies schist and quartzite this con- 

 glomerate should be regarded as " intra-formational," i. e., as 

 resulting from the slight and temporary elevation of part of an 

 Algonkian sandstone above sea-level but not from a general 

 great unconformity. The metamorphism that altered the 

 bedded quartz sandstone into quartzite must also have altered 

 the sandstone of the pebbles of the conglomerate into quartz- 

 ite ; and this metamorphism must have been that which accom- 

 panied the post- Algonkian movement. 



Since the completion of this paper the Bulletin of the Geol. 

 Society of America for March, 1916, has appeared containing a 

 brief abstract (p. 101) of a paper by C. E. Gordon, entitled 

 " Some structural features in the Green Mountain belt of 

 rocks." In this paper he refers to having observed in certain 

 places an east-west trend in the foliation of the ancient gneisses. 



Pittsfield, Mass., May 4, 1916. 



* See Pumpelly, Eaphael, Geology of the Green Mountains in Mass. Gen- 

 eral structure and correlation. U. S. Geol. Survey, Mon. 23, p. 21, 1894. 



