472 . INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



The Bolton gneiss may be traced northeasterly, easterly and southeasterly from Worcester 

 and is foimd imderlying Oxford, Millbury, Grafton, Shrewsbury, Northboro, Berlin, Bolton, 

 Boxboro, and other towns to the northeast. It is seen from this that the Bolton gneiss makes 

 up a large portion of the eastern part of the plateau of central Massachusetts. * * * 



It is weU for us, therefore, in tracing the Bolton gneiss, to foUow a southeasterly direction 

 through the town of Millbury into the town of Sutton. * * * j^ ^]jjg direction we find 

 that the Bolton gneiss extends from Worcester through Millbury and the northwestern part of 

 Sutton; then as we go still farther to the southwest, in the vicinity of West Sutton, we become 

 aware that the rock beneath is quite different. This change from one formation to another 

 may be nicely seen about three-fourths of a mile southwest of West Sutton. 



This more ancient formation consists, generally, of a light-colored, nearly white, finely 

 grained, sugary quartzite, which is at times actinoHtic. It constitutes a comparatively narrow 

 band extending through the towns of Webster, Oxford, Sutton, Grafton, and Westboro and 

 is called the Westboro quartzite. As it dips beneath the Bolton gneiss on the western side of 

 a large antichne, the quartzite is considered older than the gneiss. In appearance this quartzite 

 reminds one of the Cambrian quartzite of western Massachusetts, though there is nothing in 

 this eastern rock to definitely fix its geological age. * * * 



We have now briefly considered the rocks from Worcester to the eastern and southeastern 

 border of the plateau of central Massachusetts, bringing out the relation of each to the succeed- 

 ing one, as far as we can. Let us next start from Worcester, and, in like manner, trace 

 the rocks westerly. Passing over the phyUite, which is found underlying the central part of 

 Worcester and which has already been considered, we find in the western part of the city the 

 Carboniferous micaceous quartzite, identical with that found east and in the midst of the Car- 

 boniferous phyllite. This arrangement of these two formations is due to folding and subsequent 

 erosion, by which the lower one is revealed alongside of the upper. 



On following this micaceous quartzite in the extreme western part of Worcester the rock 

 becomes coarser in grain and abounds in parallelly injected granite, giving an appearance that 

 frequently simulates a metamorphic conglomerate. This western and more highly metamor- 

 phosed phase of the Carboniferous quartzite has been called the Paxton schist, from its occur- 

 rence in that town, just as the similar gneissoid extension of the same quartzite on the east is 

 called the Bolton gneiss. The Paxton schist is less gneissoid than the Bolton gneiss because 

 of a smaller proportion of injected granite. 



Lying above the Paxton schist, as is shown by its upper position in an anticline in the 

 northwestern part of Worcester, is a rusty, graphitic, fibrolitic mica schist identical with the 

 rusty, graphitic, fibroUtic mica schist found in patches within the area of the Bolton gneiss. 

 This rusty schist has been crumpled into almost innumerable folds, both large and small, and 

 contains much injected granite. Lying as it does above the Paxton schist, which is a more 

 highly metamorphosed phase of the Carboniferous quartzite, this rusty schist bears the same 

 relation to the Paxton schist that the Worcester phyllite bears to the Carboniferous quartzite; 

 also on the east side of Worcester there is, in a small area, a transition from the phyllite into 

 a Uke rusty, graphitic, fibrolitic schist; this rusty schist in the western part of Worcester, called 

 the Brimfield schist, is then, a more highly metamorphosed or more coarsely crystallized phase 

 of the Worcester phyllite and belongs to the Carboniferous period. This Brimfield schist, in 

 addition to making up large areas, also occinrs in limited patches within broad areas of the 

 Paxton schist. These patches probably indicate the former extension of this upper Brimfield 

 schist and are remnants that have escaped in the profound erosion by which the rock surface 

 of this plateau has been formed. These two rocks, the Paxton and Brimfield schists, together 

 with their included eruptives, make up the plateau of central Massachusetts westerly from 

 Worcester to the boimdary as already defined. 



But these Carboniferous rocks have even a greater extension to the south and north and 

 may be traced far into Connecticut on the one hand and beyond the Massachusetts boundary 

 into New Hampshire on the other. 



