GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 499 



chloritoid minerals, and it is very probable that, as the 

 material was dried at 100° only, much imcombined moisture 

 was included in the estimation. Deducting therefore 8 

 per cent, of the water (as a rough guess), and bringing the 

 remainder to 100 again, we have the composition shown 

 in Column III. This corresponds fairly closely with some 

 analyses of chlorites. 



Oalcite and dolomite are present wherever the rock is 

 much decomposed. The crystals of spar occasionally show 

 twinning, but usually do not, so that whilst both calcite 

 and dolomite are probably present it is impossible to say 

 in what proportion. Indeed the crystals may sometimes 

 be magnesite or ankerite. 



12. Origin of the Analcite. 

 The question, as to whether the whole of the analcite 

 found in the essexite under description was of secondary 

 origin, would not have been raised but for the controversy 

 which has arisen as to the existence of evidence of 

 analcite occurring as an original constituent of igneous 

 rocks, and for the fact that this mineral does occur in the 

 Prospect rock in forms very suggestive of idiomorphism. 



Much of the analcite is undoubtedly secondary for various 

 stages of decomposition of a felspar crystal to analcite have 

 been noticed. One part of a felspar may be completely 

 altered, another part of the same crystal unaltered. The 

 alteration begins both from the inter-crystal spaces and 

 along cracks in the crystals, (see Plate XXXIX, fig. 5) both 

 in labradorite and in alkali falspar, and quite as much in 

 the former as the latter. This secondary analcite after 

 felspar is penetrated by apatite needles in the same way 

 as the original felspar (see Plate XXXIX, fig. 3), and it, 

 sometimes shows the cleavage and kaolinisation of the 

 felspar, as in specimen P. 



