416 



PEOF. J. W. JTn)D OX IHE TEETIAEY AND 



rock are free from all traces of Schillerization. The crystals of 

 augite are so large that one fills up the whole field of view. (See 



page 394.) 

 Fig. 5. Anorthite-olivine-rock (troctolite) from the top of Halival, in the Isle 

 of Eum. The rock, which constitutes a vein traversing ohvine- 

 gabbro, consists of a mass of interlacing anorthite-crystals, through 

 which are scattered grains and aggregates of grains of olivine. The 

 olivine is crowded with large stellate inclusions, and its outlines and 

 cracks are rendered black and opaque. Except in the freshness of 

 its minerals and in its finer grain, this rock exactly resembles the 

 forellenstein of Volpersdorf and the Hartz. The scale on which the 

 minerals are developed is, however, somewhat smaller. Sections of 

 the Scotch rock would resemble that of Volpersdorf if the former 

 were magnified three times more hnear than the latter. (See page 

 395.) 



6. Anorthite-augite-rock (eucrite) from Hahval, in the Isle of Rum. This 



rock, which also forms veins in the olivine-gabbros, consists of a mass 

 of interlacing anorthite-crystals, perfectly unaltered, through which 

 are distributed granular particles of an augite of a green colour, 

 which is more or less perfectly converted into diaUage by Schilleri- 

 zation. In some parts of the rock, a ferriferous enstatite is added to 

 the two principal constituents, and this mineral sometimes prevails 

 almost to the exclusion of the augite. Tliis eucrite is of a decidedly 

 granulitic structure, but other rocks of the same mineralogical con- 

 stitution are coarse-grained and distinctly granitic in structure. (See 

 page 395.) 



7. Altered, ferriferous-eustatite-rock (bastite-rock) from Oarrock Den, For- 



farsliire. This rock occurs in the midst of a great dyke of serpentine 

 which traverses the Old-Eed-Sandstone strata ; it is made up of 

 an aggregate of ferriferous-enstatite-crystals in a somewhat altered 

 condition, the whole being traversed by cracks filled with serpentinous 

 material. Except for the amount of alteration the rock has under- 

 gone, it exactly resembles the " bronzite-rock " of Kupferberg in 

 Bavaria, and of St. Stephan in Upper Stj-ria. (See page 399.) 



8. Mica-hornljlende-picrite (scyelite) from an intrusive mass near Loch 



Scye, in Caithness. A number of grains of olivine, now converted 

 into serpentine, are enclosed in crystals of pale green hornblende 

 (seen in the lower left-hand part of the shde), paramorphic after 

 augite, and of a highly magnesian biotite (seen in the upper right- 

 hand part of the slide). The biotite is, in places, darkened by nume- 

 rous inclusions arranged parallel to the basal plane, or in the direc- 

 tion of the principal cleavage. (See pages 401-407.) 



Figs. 1-6 are from Tertiary rocks. 

 Figs. 7 & 8 are from Paleozoic rocks. 



DiSCTJSSIOllf. 



Mr. Etjtley said that the points raised in the paper were so 

 numerous that it became very difficult to discuss. The twin- 

 structure in calc-spar was probably produced by pressure; but 

 with regard to triclinic felspar the case seemed different ; and he 

 felt doubtful whether aU twinning could be developed by pressure ; 

 for, if so, the same structure would be produced in orthoclase. 

 Besides, twinning usually occurred in two directions in the tri- 

 clinic felspars — namely, parallel to the basal plane and to the 

 brachypinakoid. If due simply to pressure, why should it not some- 

 times occur parallel to the macropinakoid ? The proposed term 



