GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 473 



of the mineral constituents of the rocks have been deter- 

 mined, not only by calculation from the analysis 1 in all 

 cases where one was available, but also by measurement 

 according to Rosiwal's method. In one column are given 

 the proportions of the minerals by weight after restoration 

 of the original minerals from the decomposition products 

 as far as possible. 2 This restoration naturally involves a 

 certain amount of hypothesis as to the course of decompo- 

 sition, but the figures so obtained give a much better idea 

 of the original constitution of the rock than any which 

 include the decomposition products. The minerals are 

 always placed in the order of decreasing abundance. In 

 another column in smaller type are stated the figures from 

 which the restorations are made ; namely, the proportions 

 by volume of the primary and secondary constituents, as 

 obtained directly from the figures of the measurements, 

 multiplied by their respective specific gravities, and 

 reduced to percentages. 



Series I. Reservoir Quarry. 

 Specimen B. 

 Locality: S.E. end of Reservoir Quarry, 30 cm. (12 inches) 

 below junction with shale. 



Megascopic Description. Colour dark bluish-grey, with 

 typical basaltic appearance ; light reddish-brown on decom- 

 posed joint surfaces. Fracture subconchoidal. 



Microscopic Description. Texture : Crystallinity non- 

 crystalline ; grainsize, porphyritic in olivine (phenocrysts 

 0*15 to 2 mm. long), base very fine and even grained (about 

 0*12 mm.); fabric pilotaxitic. 3 



1 The method of calculation is fully explained in Appendix I. 



2 For the method adopted in restoring the original constituents of the 

 rocks see Appendix II. 



3 The term pilotaxitic is u?ed throughout this paper in a sense perhaps 

 rather more extended than the original definition of Rosenbusch. It is 

 applied to any rock of whatever grainsize in which idiomorphic prisms 

 or tables of felspar lie in all directions, the interstices being filled up 

 with grains of ferro-magnesian minerals, both idiomorphic and not, 

 having no parallel optic orientation. 



