FOREIGN WORK AMONGST THE OLDER ROCKS. 13 



we remember the almost hopeless state of confusion into 

 which the classification of the older rocks of America was 

 thrown during the raging of this controversy, it is a matter of 

 congratulation to find, as shown by this and other papers to 

 be presently noticed, that the strata of this complicated 

 region have been reduced to order by more detailed work. 

 The author of the present memoir, who notices the region 

 in another paper (8), describes the structure of the Green 

 Mountains as consisting of " a series of sharp compressed 

 folds striking approximately north and south, and overturned 

 to the west in most localities, so that induced schistosity 

 and stratification dip eastward". He uses the now familiar 

 term " Algonkian " for those Precambrian rocks which lie 

 above the " Archaean " rocks, and separates them into two 

 series, the upper or Mendon (which underlies the Olenellus 

 quartzite), and the lower or Mount Holly series. As the 

 uppermost rocks of the Mendon series consist ot mica 

 schists succeeded with apparent conformity by the quartzites, 

 "it must be left for future work to determine beyond dis- 

 pute the relations of the series immediately below the 

 Olenellus zone to the quartzite, whether the rocks are con- 

 formable or unconformable ; if the former, whether the 

 delimitation of the Lower Cambrian shall be placed above 

 the mica schist or below it. Tentatively, the series just 

 below the quartzite, the mica schist at the top, and the 

 conglomerate at the bottom, will be considered wholly or in 

 part of Algonkian age." The variations in the characters 

 of the Mendon series are mainly oi interest to the petro- 

 grapher, but where least altered, their clastic character is 

 evident, and sandstones and conglomerates occur. In the 

 typical section the series is about 1 300 feet thick, but in 

 the northern part of the State it is much more developed. 



The core of the Green Mountains is composed of rocks 

 belonging to the Mount Holly series, a more variable and 

 more metamorphosed set of stratified rocks than those of 

 the Mendon series, but " a description of the different con- 

 secutive members of the series cannot be given, as the 

 rocks are too variable in character, and dynamic action has 

 involved them in great complications". Amongst the 



