Dodge.] 390 [February 3, 



to these words. The present tendency seems to be to refer to some 

 of the less familiar attendant circumstances of sedimentation, many 

 effects hitherto supposed to be the result of " plutonic " agencies. 



Throughout the crystalline area of this vicinity there are immense 

 masses of hornblendic rock, diorite and diabase, usually cryptocrys- 

 talline, in which no indication of sedimentary origin can be traced. I 

 can suggest no test by which to distinguish among them; yet, while 

 the theory of subaqueous accumulation by chemical forces seems, in 

 many respects, to be the most satisfactory explanation offered of their 

 existence and characters, in appearance and composition they often 

 bear the closest resemblance to certain eruptives of the stratified 

 group of this vicinity, which undoubtedly had, in some cases, the same 

 origin, unless they were derived from among these crystallines them- 

 selves. 



The '' sienites " consist, for the most part, of quartz and felspar, 

 with little (often no) hornblende. The abundance of quartz seems to 

 point rather to metamorphic than igneous condition, and there is 

 probably no doubt that they are chiefly of sedimentary origin. ]STot 

 unfrequently there are grains of amethystine quartz. The felspar is 

 usually grayish, varying to blue in the favorite Quincy building stone. 

 This weathers to a light or darker reddish-brown, and the change often 

 penetrates to a great distance along the joints which cleave the rock. 

 In certain districts the felspar is often bright red. Another variety 

 particularly common in Waltham, Weston, Natick and Dover, and 

 the neighboring towns, is larger grained, and consists of red, white or 

 green felspar with less quartz. This often occurs as compact red fel- 

 spar; sometimes as a large grained, whitish (often greenish), fel- 

 spathic rock with minute specks, or rarely plates, of mica. There is 

 also a red and white granulite, and a white granular eurite. The 

 hornblende is most abundant, as a rule, in the finer grained rocks. 

 In fact, nearly all possible combinations of the elements in different 

 proportions may be found, and in many conditions of each, large and 

 small. The varieties appear to pass into each other so commonly 

 that they can not be safely separated as occupying distinct areas. 



The term " porphyry " as used in the State Geological Report was 

 applied rather indiscriminately to several widely different varieties of 

 felspathic rock, (a.) The most familiar of these is the felsite por- 

 phyry of Lynn and Saugus. This varies from flesh-color to deep red, 

 purple and black, and from a jaspery and homogeneous texture to 

 granular, either red or white. It often has pale felspar crystals con- 



