444 STRATIGRAPHICAL GEOLOGY. 



The relations of this group of strata are best seen in a section (Fig. 71) 

 from about the mouth of Couche's brook, a tributary of Fall river from 

 the west, across the supposed eastward repetition of the Williams quartzite 

 to Purple's slate quarry, and beyond. The Cambrian slates are believed 

 to dip about 50° S. E. along the west end of the section, beneath the allu- 

 vium of Fall river. The quartzite seems to stand vertical, and then to dip 

 south-east. The wrinkled schists cover it at the school-house, and dip 35° 

 S. 40° E. On the ridge west of Purple's slate quarry the dip is about 

 12° S. E. At the quarry the dip is about 20° S. E. About a stone's 

 throw to the south-east succeeds hornblende schist, dipping 70° S. E. 

 At the house are the gneissic quartzites, dipping about 30° S. E. At 

 No. 68 the dip is 30'^ S. 40" E., and the hornblende beyond is supposed 

 to lie in about the same position. 



Our observations seem to establish a direct continuity of exposures of 

 these pimpled staurolite mica schists from the east foot of the Williams 

 hill, overlying the quartzites to Chesterfield, and consequently with the 

 extensive area of the Coos groups reaching to Haverhill. Hence it con- 

 firms the first part of the previous conclusion of Prof. Dana,* that the 

 " Coos group, which is but the northern continuation of the same [Ber- 

 nardston] series, is, if correctly traced out, also Helderberg." The refer- 

 ence here is to the mica schist division of the group. 



6. Helderberg Limestone. 



There is but a single exposure of this limestone, and therefore I will 

 carefully describe the jDOsition of the rocks associated with it. It occurs 

 upon the east slope of a hill a mile north-west from Bernardston railroad 

 station, and not over a fourth of a mile distant from ledges of the Trias- 

 sic sandstone. We enter through a gateway of Mr. Williams's from the 

 carriage-road, and encounter immediately a small knob of the blackish, 

 pimpled clay slate, dipping 30° S. 48° E. This is the western edge of 

 the Coos slates ; and in the field farther north it occurs with a high dip 

 to the north-west. Next succeeds the quartzite, dipping at first 29° S. 

 35° E., and then 25° S. 50° E. This is at a quarry. A hundred feet to 

 the south of the section line, and nearer the limestone, the quartz is more 

 vitreous, and dips 52° S. 40° E. In the field north of the section, in 



* Amer. your. Set., iii, vol. vi, p. 349. 



