GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 501 



Frequently allotriomorphic masses of analcite include 

 perfectly fresh felspar as well as apatite and fresh augite, 

 and are surrounded by apparently fresh rock. These 

 analcite masses often exhibit the weak anomalous double 

 refraction which has been noted in other rocks by various 

 authors, and which is no doubt due to the unequal strain 

 set up by local changes of volume in the surrounding 

 minerals as they continue to decompose. In some areas 

 of analcite there may also be seen a patch colored a deep 

 dirty brown by transmitted light which shades off into the 

 surrounding analcite without a sharp boundary. Similar 

 patches have been noted by Mr. G. W. Card in the analcite 

 bearing basalts of the Sydney district, and he suggests that 

 they may be caused by alteration of the analcite. 1 Examin- 

 ation with a high power shows clearly that they consist of 

 analcite closely packed with inclusions; but the nature of 

 the latter we were unable to determine. The frequent 

 occurrence of these patches in the centre of an analcite 

 mass does not lend support to the view that they are formed 

 by its alteration ; and we are not able to offer any opinion 

 as to their origin. In other places the analcite appears to 

 play mainly an interstitial role. 



There can be no doubt whatever that the greater part 

 of the analcite is secondary; and it appears to us, after 

 careful consideration, that all the many forms in which the 

 mineral occurs at Prospect are capable of explanation on 

 the hypothesis that it is a product of decomposition. If 

 the analcite, which occurs in rounded or nearly icliomorphic 

 masses were original, we should be forced to conclude 

 that it crystallised before the felspar, in part at least # 

 Cross and Pirsson, however, consider analcite the last 

 substance to crystallise in the supposed analcite-bearing 

 igneous rocks. 2 On the other hand, were there any analcite 



1 Eecords of the Geological Survey of N.S.W., Vol. vu, (pt. 2) p. 93. 



2 The Monchiquites or Analcite-bearing Igneous Eocks, by L. V. 

 Pirsson, Journ. of Geol., Vol. iv, (1896) No. 5. 



