252 THE CANADIAN NATURALIST. [Yol. ix. 



If now we pass to C, the uralitic portion of the crystal, the 



chanires are much more strikins; as will be seen from the follow- 



ins: analysis : 



C. 



Silica 52-823 



Alumina 3-215 



Ferric oxide 2-067 



Ferrous oxide 2-709 



Manganous oxide 0-276 



Lime 15-389 



Magnesia 19-042 



Potash 0-686 



Soda 0-898 



Loss on ignition 2-403 



99-508 



The specific gravity in this case was only 3.003. Comparing 

 C. with A. and B. we find that the lime is diminished by about 

 nine per cent., while there is a gain of about four and a-haif per 

 cent, of magnesia. The ratio of loss and gain, however, is not 

 that of the molecular weights of lime and magnesia ; that is to 

 say, for a molecule of lime lost a molecule of magnesia has not 

 been gained. A portion of lime has been lost without its place 

 being taken by magnesia. At the same time there is a slight 

 increase of silica relatively to the other constituents, and, as 

 would be expected, a decrease in density. 



It is well known that pyroxene commonly contains more lime 

 and less magnesia than hornblende, and in the present case loss 

 of lime and gain of magnesia would appear to be the principal 

 cause determining the change to hornblende. The larger pro- 

 portion of alkalies in the uraltic or hornblendic proportion of the 

 crystal is also worthy of note, because hornblende is commonly 

 richer in alkalies than pyroxene. On the other hand, it is in- 

 teresting to observe that there is less alumina in the hornblendic 

 product than in the original pyroxene, for, as a rule, hornblende 

 is apt to contain more alumina than pyroxene. This subject has 

 recently been discussed by Mr. G. W. Hawes in his valuable 

 report on the mineralogy and lithology of New Hampshire. He 

 there gives some interesting analyses to illustrate the diflerences 

 in the composition of pyroxene and hornblende, and seems to 

 regard preponderance of alumina as the principal cause determin- 

 ing the formation of the latter species. At the same time, how- 

 ever, he does not lose sight of the fact that pyroxene usually 

 contains more lime and less alkalies thuu hornblende. 



