nORN EXPEDITION — PETROLOflY. 89 



crystalline bounclarios, it must be placed amongst diallages, in virtue of the 

 development, especially in the neighbourhood of cracks, Plate IT., Fig. 3, of a fibrous 

 structure pai-allel to the vertical axis, and the comparatively low extinetinn angles, 

 which seldom exceed 31°. On the orthodiagonal section the optical sign is positive. 

 The felspars have fairly high extinction angles averaging about 26° or 27°, which 

 indicates a composition near that of l)yto\vnite. Magnetite is rather plentiful, and 

 in the form of large irregular grains. Fine acicular crystals of apatite occur in the 

 plagioclase, and traverse small grains of secondary quartz. There are present, as 

 additional secondary minerals, small pieces of a greenish-yellow mica, witli strong 

 ab-sorption, surrounded by a small quantity of a greenish decomposition product 

 and a little pyrites. Some of the felspars have been replaced by an aggregate 

 of minute pai'ticlcs of calcite. This rock occurs, between Slip panel Gap and 

 Ellery Ci'eek, intruding gneiss. 



Dolepite. No. 90. Plate IV., Fig. U. Sp. gr. 3-03. 



This rock is very similar to Nos. 195 and 179. It is a coarse-grained rock 

 consisting of plagioclase, augite, olivine, aiid magnetite. 



The felspar crystals often present the two (al)ite and pericline) types of 

 twinning. There is strong evidence of zoning, which is made apparent )jy the 

 " shadowy " extinction. The angles of extinction vary from tlie extei'ior to the 

 centre of the crystals, the external portions often having an almost straight 

 extinction, while the central [xirtions extinguish at an angle of about 31°. From 

 this we may conclude that the outer portions of the felspar crystals belong to the 

 soda end of the soda-lime series, while the central portions belong to the lime end. 



The ti-ansition from felspar of one composition represented liy tlu; high extinc- 

 tion angles to that of a very diflerent composition represented by the low extinc- 

 tion angles is not perfectly progressive. The process of the separating out of 

 felspar of a gradually changing composition seems to have received a check at a 

 given stage, when for a certain space of time felspar of a fixed composition seems 

 to have crystallised out. This was the most stable felspar under the circumstances, 

 tliat is, that most able to maintain a fixed composition in spite of tlie changes 

 taking place in the magma. The most stable felspai' in this case appeai-s to have 

 been one with an extinction angle between 18° and 28°. 



The augite in thin sections is of a pale brownish-yellow colour. 



Olivine, Plate IV., Fig. G/', occurs in irregularly-shaped grains which are 

 much cracked, the cracks being to a great extent filled with line grains of 



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