107 



over, they embrace all the principal varieties of structure 

 characterizing the great mass of the petrosilex, and in the case 

 of the schistose structure, especially, the identity seems to be 

 perfect. The petrosilicious rocks are of two colors, white and 

 pink, and the latter, particularly, makes decided approaches to 

 granite. The felsite is often inseparable from diorite. The 

 quartzite is usually gray and more or less micaceous, but there 

 are large masses of nearly vitreous quartz of a snow-white 

 color. The quartzites lie mainly to the south, or under the 

 diorites, the series becoming more hornblendic northward. 



The rocks of this large area are well exposed, also, at several 

 points on the Boston and Lowell Eailroad. Here, interstrati- 

 fied, we have distinct argillite, diorite of many different text- 

 ures, felsite, gneiss, and various nondescript rocks transitional 

 between those mentioned. The strata are much contorted 

 in some places, and yet they seem to be entirely conformable. 

 The normal dip is N. 30° W. 70°-80°. The argillite is fissile, 

 with shining surfaces. The felsite near the chemical works in 

 North Woburn is very similar to that occurring at Dungeon 

 Rock in Lynn. These rocks probably extend north to the 

 Montalban boundary, and south-west to Concord, and they 

 have been so represented on previous maps ; but the wide- 

 spread drift deposits make this point difficult to determine, and 

 I deemed it best not to map beyond my observations in a region 

 so complicated as this. 



The long Huronian tongue penetrating the Montalban forma- 

 tion south-westerly from Concord, lies directly in the trend of 

 the area just described, and, since it is entirely composed of 

 similar stratified rocks, the probability is strong that these two 

 areas are really one. Indeed, I have no doubt that these dio- 

 ritic rocks do form the entire north-western border of the Hu- 

 ronian system, from North Reading to Concord ; the only 

 question is whether they are always stratified or not, and it is 

 probable that they are. I have made but one section across 

 the Concord and Westborough band, viz., in Marlboro'; but, 

 according to Prof. Hitchcock, the rocks are the same at all 



