170 Alfred Marker — Various Crystalline Rocks. 



cvystal-grains up to one-tenth of an inch in diameter, showing 

 marked pleochroism in pink and green tints. Cleavages parallel to 

 the two pinacoids occur, in addition to the usual prismatic cleavage. 

 There is often a minute "schiller" structure, apparently parallel to 

 the brachypinacoid, besides larger inclusions, which have the same 

 orientation, and appear to be of the nature of I'homboidal cavities 

 filled with some dark brown ferruginous material. The absorption- 

 colours, so far as can be made out in random sections, are, parallel to 

 a=:a ... rose-pink ; 

 yQ=b ... nearly colourless ; 

 <Y=c ... pale apple-green. 



The diallage is in colourless, irregular plates, often slightly bent. 

 It shows rather broad bands following the orthopinacoidal parting 

 and corresponding to the "solution-planes." These bands are 

 occupied sometimes by a colourless and highly birefringent crystalline 

 mineral like calcite, sometimes by brown or greenish-brown matter. 

 There is a strong cleavage always making a high angle with these 

 bands and probably parallel to the basal plane ; this, too, seems to 

 be a direction of chemical weakness, though a less marked one. 

 Less pronounced cleavages correspond, no doubt, to the prism-faces. 



The only other mineral which can be considered original is a 

 deep brown, pleochroic hornblende, which occurs rarely, in small 

 scraps, intergrown with the diallage near its contact with the 

 hypersthene. There is no trace of felspar or olivine, nor any- 

 original iron-ores or other accessory minerals. 



Though often quite fresh, the hypersthene sometimes shows 

 alteration into a minutely granular aggregate which gives high 

 polarization-tints. The change begins in veins following cleavage 

 cracks, but may destroy the whole of a crystal, so that nothing is 

 left of the original structure of the hypersthene except the dark- 

 brown schiller-inclusions mentioned above. Into the spaces thus 

 occupied by the obscure alteration -product, project jagged fringes 

 of colourless, brightly-polarizing hornblende, which, although formed 

 at the expense of the rhombic pyroxene, grows as an extension of 

 the diallage, towards which it is oriented in the usual crystal- 

 lographic relation. 



(ii.) Eclogite from Port Tana in the North of Norway. 



This rock, collected by Mr. E. H. Solly, shows in a hand-specimen 

 plenty of red garnets imbedded in a yellowish-green crystalline 

 mass in which pyroxene (omphacite) is evidently the dominant 

 constituent. 



In a slice [962] the garnet shows an irregularly rounded outline 

 and a faint pinkish tinge. Some of the crystals are distinctly 

 double-refracting. This character is not associated with any poly- 

 synthetic structure, and may be due to internal strain ; it is some- 

 times more marked in the vicinity of certain inclusions. A constant 

 feature is the presence of countless minute rods accurately alligned 

 in three directions pai'allel to the three crystallographic axes. They 

 belong to a clearly double-refracting mineral, and give extinction- 



