BY W. N. BENSONj \V. S. llUX^ AND \V. It. BROWNE. 417 



parallel iiitergrowths of the two are seen. Slight alteration is noticed in the 

 hyperstheue, but the augite is quite fresh. Irregular grains of magnetite or 

 ilinenite are fairly numerous. The groundmass is hypohyaline and micropor- 

 phyritic, small grains and crystals of plagioclase and pyroxene being set in a 

 base composed of greyish-brown glass with much mierolitic augite, felspar" and 

 magnetite. Round the microlites there is often exhibited the characteristic border 

 of glass of lighter colour than normal. 



The chemical composition of the pitchstone may be gathered from the analysis 

 of a spoeimeu from Portion IIG. 



1. II. III. IV. 



SiO- lJU.2(i 58. 7i) 61.17 59.48 



AI2O3' 16.46 17.51 17.74 17.38 



TeaOa 1.15 2.11 1.78 2.96 



FeO 4.87 3.87 3.51 3.67 



MgO 3.09 2.23 2.70 3.28 



CaO 5.25 6.18 5.90 6.61 



NaaO 4.23 4.84 3.79 3.41 



K2O 0.98 0.68 1.71 1.64 



H2O+ .... 2.22 2.61 , 



H2O- .... 0.22 0.71 ; "-^^ "-'^ 



Ti02 0.84 1.21 0.45 0.48 



P2O5 0.29 — 0.14 0.20 



CO2 abs. tr. — — 



S .; 0.03 _ _ _ 



Cr208 abs. — — — 



MnO 0.08 — 0.12 0.15 



BaO abs. — 0.06 — 



SrO abs. — — — 



99.97 100.74 99.96 100.00 



I. Andesitie pitchstone, portion 116, Currabubula. Anal. W. N. Benson. 

 II.* Andesitie pitchstone, Pokolbin. Anal. AV. R. Browne. 

 Ill.t Hypersthene andesite, Lassen Park, California. Anal. Hillebrand. 

 IV. t Osann's Average of hypersthene andesite. 



It will be seen that there is a very close correspondence between the Curra- 

 bubula and Pokolbin rocks, quite sufficient to place their consanguinity beyond 

 aU doul)t. In mag-nesia and lime both rocks are somewhat lower than usual, and 

 though the total of the alkalies is normal, soda is conspicuously high and potash 

 correspondingly low. 



The pitchstone from the small intrusion revealed in the railway cutting in 

 Portion 34 differs from that of the larger masses in having a certain amount of 

 perlitic cracking, while the phenocrysts are smaller and less numerous. In some 

 parts of the sill a certain amount of alteration has occurred, rendering the rock 

 lustreless and giving it a purplish-gTey colour. In thin section this peculiarity 

 is seen to be due in part at least to the separation of haematite from the base. 



•See (52), p.404. 



tQuoted by Tclflmg.s. — Igneous Rocks, Vol. ii., p.44:i. 



IQuoted by Daly. — Igneous Rocks and Their Origin. 



