548 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



Sum of mineral weights 97*78. 



Sum of analysis 97*33. Excess °' 45 - 



Composition of Plagioclase : AbiosAn 7 = Abi 5 An la 



Appendix II. 



Restoration of the Original Composition of the 

 Decomposed Rocks. 



The secondary constituents of igneous rocks falJ into two 

 classes: the products of incipient alteration, and the 

 minerals resulting from advanced decomposition. The 

 former, such as kaolin and scapolite, are without influence 

 on the boundaries of the crystals in which they occur, and 

 in the Rosiwal measurements they count as parts of those 

 crystals and are wholly neglected. Secondary minerals of 

 the latter kind, on the other hand, frequently transgress 

 the original limits of the crystals from which they are 

 formed ; so that in measuring a rock there is no option but 

 to reckon them separately, and allow for them afterwards. 

 The most abundant products of advanced alteration in the 

 Prospect rocks are analcite, serpentine, and chlorite, with 

 which we may include the chloritoid, owing both to its 

 similarity in composition, and to the difficulty of separating 

 the two under the microscope. Stilbite and calcite, or 

 dolomite, are comparatively rare as defined masses ; but 

 calcite probably forms some proportion, though often a 

 small one, of most of the masses of chlorite. Thus the 

 only secondary minerals which have to be broken up and 

 allotted to original minerals are analcite, serpentine, chlor- 

 ite and a little calcite, with occasionally some stilbite. 



The method here adopted is first to calculate the quantity 

 of augite and olivine which would have been required to 

 form the weights found of chlorite and serpentine respect- 

 ively. The quantity of felspar lost by decomposition is 

 then determined by reckoning the volume of analcite found 

 as felspar, and by adding to this the weight of felspar 



