Speight. — On an Occurrence of Olivine- andesite. 371 



small quantities of limonite, evidently clue to the action of 

 atmospheric agencies or percolating water. 



II. Porphyritic Minerals. 

 1. Felspar. — This is by far the most prominent porphy- 

 ritic mineral, as it can easily be distinguished by the naked eye, 

 while the other minerals can only be distinguished microscopi- 

 cally. It is of all sizes, from almost microscopic dimensions 

 to crystals half an inch in length. The felspar is almost 

 wholly labradorite, but oligoclase is' present to a small amount. 

 The evidence for the species of felspar is as follows : — 



(i.) Chemical Analysis. 



Portions of the rock were taken and broken in order to 

 obtain pieces of the felspar. These were tested chemically, — 



(a.) With the flame, showing well-marked calcium, obscur- 

 ing the sodium to a certain extent, but a trace of 

 potassium appeared when the flame was observed 

 through blue glass. 



(b.) The presence of CaO was shown by precipitation with 

 ammonium oxalate, when a decided precipitate ap- 

 peared. 



(c.) By the action of strong hydrochloric acid the mineral 

 was partly dissolved. 



(d.) By quantitative analysis the 

 obtained : — 



Loss on ignition 



Silica 



Alumina 



Ferric oxide 



Lime 



Soda 



Potash ... 



The amount of ferric oxide is probably accounted for by 

 the presence of minute inclusions of limonite. 



The large value of the CaO in No. I. is probably due to in- 

 correct weighing, as the calcium was estimated as CaO after 

 ignition. As this analysis was almost qualitative, extreme care 

 was not taken. The second determination is probably exact. 

 All the alkali present was estimated as soda, since the flame- 

 test gave such a slight amount of potash. By this analysis 

 the felspar appears to be labradorite, but the amount of CaO 

 is slightly too low for a typical specimen. This may be due to 

 the presence of a small quantity of oligoclase, which subse- 

 quent determination shows to be present. 



