L22 T. X. Dali —Algonkian-Cambrian Boundary. 



Another structural feature of the region is that the pre- 

 Cauibrian gneiss exceptionally has two foliations with strikes 

 corresponding to that of the pre-Cambrian and Cambrian 

 respectively. Thus on the east base of Ludlow Mountain 

 some granite-gneiss ledges have coarse plications striking 

 X. 50 '-90° W. but a hue foliation striking N. 20°-30° E. 



A pre-Cambrian conglomerate in North Sherburne consists 

 of pebbles arranged in small beds in a schistose cement. The 

 pebble beds strike N. 00° W. with the granite-gneiss but the 

 slip-cleavage foliation of the cement strikes N. 10° W. with 

 the Cambrian schist of the region. 



A bed of quartzite 40-50 ft. thick, in the mass east of North 

 Sherburne, has the typical pre-Cambrian strike of N. 70° W. 

 but curves around sharply to strike N. 30°-50° E. with the 

 Cambrian beds. The bending has resulted in much minor 

 faulting. This occurrence indicates that the conformity of 

 strike existing in so many places between the pre-Cambrian 

 and Cambrian may be due to changes produced by the post- 

 Ordovician movement in the pre-Cambrian structure. 



Origin and composition. — The Cambrian rocks along the 

 boundary studied include: metamorphic arkose, quartzite, al- 

 oitic muscovite, and muscovite-biotite, also albitic-chlorite schist, 

 and dolomite, both granular and twinned. The quartzite 

 generally includes some beds of sericite schist. Black tourma- 

 line is abundant and in places associated with pegmatite. 



The pre-Cambrian rocks include various granite-gneisses, 

 aplite gneiss, metamorphic arkoses, quartzite, conglomerate 

 with pebbles of quartzite, albitic sericite schist and graphitic 

 sericite schist. The age determination of these Algonkian 

 sedimentaries is based entirely upon their strike being con- 

 formable to that of the underlying granite-gneiss and uncon- 

 formable to the adjacent overlying Cambrian beds. Some of 

 these Algonkian schists are petrographically identical with 

 Cambrian and Ordovician ones of the Green Mountain region. 



The arkose and quartzite conglomerate call for more detailed 

 description. One of the marked types of arkose is a medium 

 to dark grayish rock, in places with lighter grayish less micace- 

 ous bands. It consists of more or less angular grains of quartz, 

 of multiple-twinned plagioclase, of microperthite, in places of 

 microcline, in a cement of muscovite, chlorite, biotite and epi- 

 dote with accessory zircon, apatite, pyrite, limonite. The 

 quartzite conglomerate in North Sherburne measures roughly 

 not less than 275 ft. in thickness and is separated from an 

 underlying 40-50 ft. thick bed of quartzite by a hundred feet 

 or more of albitic sericite-chlorite schist. At the contact If 

 miles N.NW. of Sherburne village the Algonkian schist and 

 arkose include two beds of quartzite, 10-15 and 3-10 ft. thick. 



