GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 497 



11. Characters of the Secondary Minerals. 



The following is a complete list of the alteration products 

 which we have recognised in the Prospect essexite : — 

 Derived from felspar : — analcite, kaolin, scapolite, 



stilbite. 

 Derived mainly from ferric minerals : — serpentine, 

 chlorite, chloritoid, calcite, dolomite (or ankerite). 



Pyrite is also present, and some of it at least is probably 

 secondary; possibly all of it is secondary, and its inclusion 

 in the list of primary minerals is hypothetical. 



The origin of the analcite will be discussed in the next 

 section; the kaolin occurring in the alkali felspars, oligo- 

 clase and perhaps andesine, calls for no remark, and we 

 may therefore pass to the scapolite. This mineral was 

 identified in thin sections, and it occurs widely in the mass 

 as agranular filling of widened cracks in the labradorite. 

 It is the second product to appear in the alteration of the 

 rock, analcite being the first, but the scapolite is itself 

 decomposed as alteration proceeds, whilst the analcite 

 persists. 



The stilbite does not appear until alteratiou has pro- 

 gressed so far that there is a good deal of analcite formed, 

 and the olivine is almost wholly changed to serpentine ; 

 and, as several altered specimens are without it, it is 

 apparently not always formed. When present it occurs as 

 sheaf or fan-like masses of long fibres. Each mass has but 

 one centre of radiation, and replaces a felspar crystal, 

 pseudomorphing its faces often on one side only. 



The serpentine and chloritoid are both products of the 

 alteration of the augite. The former is of dull bluish colour 

 and shows the familiar grey to inky blue polarisation tints. 

 The latter occurs in masses of radially arranged bundles 

 of fibres resembling secondary spherulites. It is grass- 

 green in colour, and strongly pleochroic from yellow (c) to 



