HGOSAC MOUNTAIN. 



101 



without passing into it, and the quartzite has a curious thickening of its 

 layers where the dike joins it, as though there had been a hollow, owing to 

 erosion of the dike before deposition of quartzite. It seems therefore to 

 show the most perfect unconformity between the granitoid gneiss and the 

 overlying quartzite, although the lines of structure of both rocks are parallel. 

 (See Figs. 28 and 29.) We can trace this contact northward for a quarter of 

 a mile or more; the quartzite is interbanded with very feldspathic gneisses, 

 the whole forming quite a thick series. The rocks dip east (43° east, strike 



Fig. 29. — Contact of granitoid gneiss and quartzite; same locality as 2*, looking east, showing the quartzite nearer. 

 The dike was found, by digging, to lie against the quartzite without passing into it, and the quartzite shows a cirri* 

 ous lenticular thickening just in the line of the dike, as though there had been a depression there at the time of deposit. 



north 40° east) and so does the structure of the granitoid gneiss. Between 

 this point and the quartzite above North Adams one outcrop of quartzite 

 conglomerate has been found in place, strike north 45° east, dip 30° east. 

 There seems therefore no doubt that this series of quartzites and gneisses, 

 lying on the granitoid gneiss without a fault, are the same as the quartzite 

 at North Adams, •_' miles off: they have the same strike and dip and lie 

 on the same i-ock, and a glance at the map will show that the line of strike 

 runs from one to the other. We have here then the second proof that the 



