EASTEKN MASSACHUSETTS 



427 



has been over this ground, can doubt that these different rocks are stratigraphi- 

 cally inseparable. Ah)Bg the eastern border of this area of stratified rocks, 

 one can find beautiful examples of the passage of stratified diorite and horn- 

 blende slate into the eruptive diorite and fine grained hornblendic granite. 



There is nothing abrupt aliout these transitions, the gradation is perfect 



About four miles farther northwest, in Heading, .... we find the rocks 



sliading insensibly into eruptive diorite and granite The large area of 



diorite stretching from Stoneham to Weston includes numerous small patches 

 of stratified rocks — hornblende slate and petrosilex They pass fre- 

 quently into the enclosing rock, showing that they are mere remnants of the 

 stratified group, which yet preserve traces of the structure once possessed by 



the whole mass of the rock It thus becomes evident that this diorite, 



the so-called * Salem syenite,' has, like the diorites elsewhere, resulted from 

 the extreme mctamorphism of the stratified group I liavc already indi- 

 cated that much of the hornblendic granite has been derived by metamorphisui 



from felsite I have show^n that this is the probable origin of all the 



Huronian granites. It is demonstrated that the most, and probably all, of the 

 felsites of this region, are now, or were originally, stratified, and may, there- 

 fore, be logically included in the stratified group The rocks of this 



area consisted once, speaking generally, of stratified felsite, hornblende slate 

 and stratified diorite only, which were then, and are even now, so related strati- 

 graphically, and exhibit such frequent lithological transitions, as to prove 

 them to be members of one and the same unbroken series ; and that the fel- 



4 



sites, by metamorphism, have given rise to granite, the hornblende slates, in 

 like manner, to fine grained hornblendic granite approaching diorite, and the 

 stratified diorite to eruptive diorite." 



Of the Huronian formation in general it is stated (L c, pp. 27-29) : 



*' All the stratified rocks of this formation north and west of Boston, includ- 

 ing besides the so-called stratiliod group, the dolomites and stratified felsites, 



dip, with few and unimportant exceptions, to the north>vest Since the 



limits of the different rocks are marked by gradual transitionSj which precludes 

 the existence of faults of any great extent ; and since there are no apparent 

 repetitions of the different rocks, which precludes the existence of considerable 

 folds of the strata ; we are forced to the conclusion that the geographical ar- 

 rangement corresponds to the stratigraphical succession ; and the rocks to the 

 Bouth-east nnist underlie those to the north-west — the felsites must be older 



than the hornblende slates and diorites Since tlie granites are more 



intimately associated with the felsites than any other of the stratified rocks, 

 and are probably derived from them ; and since the exotic granites occur as 

 eruptives through all the stratified rock ; the inference is plain that the gran- 

 ites belong normally at the bottom of the series, and may be taken to represent 



the lower portions of the felsite The oneness of formation has been 



established ; it is proved by its petrological, its lithological, and as I have else- 



>l 





^1 



