Ill 



occurrence in the diorite formation of Marblehead, Swampscott, 

 Salem, etc. ; and the diorite itself is frequently stratified, 

 especially the quartzose portions. It is unnecessary to append a 

 list of localities where the stratification of the diorite has been 

 observed, since the student can hardly fail to meet instances 

 of this structure in any part of the area indicated. As a rule 

 these stratified patches are not sharply outlined, but are gradu- 

 ally merged with the enclosing rock, the bedding becoming less 

 and less distinct and insensibly vanishing as we proceed from 

 their centres outward. The large area of diorite stretching 

 from Stoneham to Weston includes numerous small patches 

 of stratified rocks, — diorite, hornblendic gneiss, felsite, and 

 quartzite. A few of these patches have been represented on 

 the map, but many more, probably, remain undiscovered. 

 They are usually narrow, parallel with the strike, and vary in 

 length from a few rods to a mile or more. Strike, N.E.— S.W. ; 

 dip, steep to N.W. They frequently pass into the enclosing 

 rock, showing that they are mere remnants of a vast forma- 

 tion, which yet preserve traces of the structure once possessed 

 by the whole mass of the rock. The stratified rocks compos- 

 ing these narrow bands are similar to those forming the 

 large areas already described, comprising, chiefly, diorite, 

 felsite, and quartzite ; it is probable, however, that they 

 generally include more quartzite and less diorite. This is 

 precisely what must result when such a lithologic series as 

 that in question is metamorphosed ; the hornblendic rocks 

 are most susceptible of alteration and are the first to yield, 

 those containing the largest proportion of hornblende, the 

 diorites, being altered more readily and extensively than 

 those which are largely feldspathic or quartzose. Felsite and 

 petrosilex may be changed, but less easily, into feldspathic 

 diorite or granite ; but the quartzite is almost incapable of alter- 

 ation. Excessive disturbance may obliterate the lines of bed- 

 ding, and the rock may become more or less vitreous, but it is 

 a stratified quartzite still ; quartzite being the last rock that we 

 can expect to find playing the role of an eruptive. Conse- 



