270 



lex changing to green near the joints. In many cases, doubtless, 

 the change is to kaolinite rather than pinite, but not always. 



The clearest example of the formation of pinite from petro- 

 silex which this region affords, so far as I have observed, is 

 that already referred to on Central Avenue in Milton. The 

 color of the unaltered petrosilex in this case is dark purple, 

 and the pea-green pinite occurs in it in the form of irregular 

 and ill-defined masses which seem to have their major axes 

 normal to the surface of the ledge. Closer observation shows 

 that they follow the jointing of the petrosilex ; each joint being 

 bordered on either side by pinite which shows a gradual pas- 

 sage into normal petrosilex at a distance of a few inches. The 

 rather limited exposure is best in the vertical direction ; and, 

 tracing one of the pinite-bordered joints downward, it seems 

 plain that the zone of this material is broadest and best marked 

 near the surface, becoming narrower below, and almost entirely 

 fading out at a depth of a few feet. 



This peculiar disposition of the pinite in the petrosilex evi- 

 dently leaves us no option but to believe that here at least it 

 is a decomposition-product, and that percolating atmospheric 

 water, for which the joints have afforded channels, has been the 

 chief agent in its formation. The conglomerate cannot have 

 been long removed from this part of the petrosilex ; and hence 

 this surface is clearly an ancient one ; and I take it that we 

 have here an example of pre-primordial decomposition. 



(2.) The composition of a characteristic specimen of the 



pinite, taken from its original position in this ledge, is shown 



by the following analysis, made by Miss E. M. Walton : — 



Si 2 57.924 



Al s 3 23.739 



Fe O 2.826 



K 2 O 4,560 



Na 2 O 5.283 



H 2 O 3.142 



Mn O 1.443 



Cr and Mg traces 



98.917 



