Geolocjy of Broadmeadoivs. 165 



burgose. The high percentage of combined water, 2.60 per 

 cent., is notable, and finds expression in the large quantity of 

 brown glass in the rock. 



Petrngra/phical Characters. 



Specimen No. 24 was obtained from the older basalt quarry. 

 Megascopically it is a very dark, hard and dense basalt which is 

 rather brittle under the hammer. Crystals of olivine and a very 

 occasional crystal of felspar can be observed in the hand speci- 

 men scattered throughout a dark-coloured matrix. The outer 

 shell of the specimen contains no phenocrysts, and is distinctly 

 more tachylitic. Microscopically the rock consists mainly of 

 phenocrysts of olivine set with felspar laths and grains of 

 augite in a ground mass of brown glass. 



The olivine phenocrysts for the most part are clear and 

 unaltered in imperfectly shaped crystals, though some good 

 hexagonal and octagonal outlines may be seen. Some of these 

 show simple and interpenetrative twinning. Stages in the altera- 

 tion of the olivine to a yellow-brown mineral may be observed. 

 This red-brown alteration product is more often seen than the 

 intermediate stages, indicating that the change, once started, 

 takes place very rapidly. This red-brown mineral corresponds 

 with iddingsite, except that it is isotropic in the complete 

 crystal. In the half-way stages it extinguishes in the same 

 position as the olivine, and corresponds exactly with iddingsite. 



The felspar is present in two distinct types. Most of the 

 laths show lamellar twinning, and, in the measurement of the 

 angle of extinction of the lamellae in a large number of sections, 

 the maximum observed was 42 deg., while several values of 

 35 deg. were obtained. This felspar, then, according to the 

 diagrams of Michel-Levy, is labradorite with the composition 

 Ab^ An^. One large phenocryst of this felspar is present. It 

 is corroded by the magma and is crowded by black inclusions. 

 It show^s but a trace of the lamellae, and also shows cleavage, 

 and is therefore a section almost parallel to (010). Its extinc- 

 tion angle measured from the traice of cleavage is 41 deg., show- 

 ing that in composition it approaches pure anorthite. In addi- 

 tion to this basic felspar there is a tabular felspar which has 



