1877.] 235 [Wadsworth. 



Street to the rock on the boundaries of the Lowell Street " green- 

 stone " dike. 



Summary and Conclusions. 



It will be seen from the foregoing that the first three classes of 

 rocks, i. <?., " diorites," " traps " and " greenstones," show various 

 degrees of alteration, from those in which the original constitutents 

 are but slightly altered to those in which viridite and some magnetite 

 form the entire mass of the rock, all of the original constituents hav- 

 ing been changed to viridite, except the magnetite, that change 

 taking place through the medium of percolating waters, and in many, 

 if not in all eases, going on at the present time. 



The " diorites " and " trap " having a common direction and com- 

 position, as well as a similar structure depending upon the width of 

 the dike, must be taken as of the same age and as the same rock. 



The "greenstones " having the same original constituents as the 

 former, excepting their greater alterations, and in some cases closely 

 resembling in internal structure the " diorites," should be taken as 

 the same rock, but of a greater age. 



The " porphyry " could come from altered trachytic rocks, but, 

 from its general association and its resemblance to parts of the 

 greenstones it may be considered (provisionally at least) as belonging 

 to the " greenstone " type, but in narrower dikes and of the same or 

 different age. 



The normal and essential constituents of these rocks were augite, 

 feldspar, and magnetite, containing as accessories apatite, olivine, 

 pyrite, hornblende, and perhaps some biotite. 



The viridite, most of the biotite, part of the hornblende, the preh- 

 nite, chalcodite and hematite, are the products of alteration. The 

 viridite is in many, if not in all cases, the intermediate or transition 

 stage through which the original constituents pass in the course of 

 their replacement by biotite, hornblende, chalcodite, etc., etc. 



Many of the rocks here described have a tendency to decay from 

 centres forming dark spots, which are often removed by percolating 

 waters leaving cavities that are afterwards filled by infiltration, and 

 in other cases the filling proceeds simultaneously with the removal. 



This process, which can be seen in every stage in some of the 

 dikes gives rise to much of the amygdaloidal appearance of these 

 rocks. 



