104 MB. C. E. TILLET OS THE GEA>T:TE-Gr>EISSES [vol. Ixxvil, 



I'vroxene is present only in small amount. It is oceasionally 

 seen to be midergoing ui-alitization : but in the majoritr of cases 

 the hornblende is of entirely new crystallization. Moreover, 

 pyroxene and amphibole may appear together with idioblastic 

 outlines. The auedte is slio-htlv srreenish. but is not sensiblv 

 pleochi-oic. Tliis pyroxene cannot be the original pyroxene of the 

 igneous rock from which the amphiboKte is derived, but has itself 

 been recrystallized. What are probably ihuenite-grains are some- 

 times developed as inclusions in the pvroxene, and are also to be 

 seen as cores to spindle-shaped grains of titanite. 



Plagioclase with quartz forms the intei-stitial material. 



The felspar occurs in irregular to rounded grains showing albite- 

 lamellation, and occasionally pericline in addition. The inverse 

 zoning characteristic of metamorphic plagioclases has also been 

 recorded, but a zonary structure is not a constant feature. 



Optically the plagioclase has the properties of oligoclase-andesine 

 and andesine. The sign of the birefringence is for the most part 

 negative, but positive grains liave been recognized. Sections per- 

 pendicular to (010) show symmetrical extinction up to 16^. 



Quartz which is intimately associated with plagioclase is to be 

 considered as the normal bye-product of the amphibole develop- 

 ment. In a further varietal type, pyroxene has completely dis- 

 appeared, and biotite and apatite are further accessories. 



The low- temperature degradation of garnet here has yielded 

 chlorite, showing the anomalous interference-tints of penninite. 

 This chlorite is developed in the numerous fractures which tra- 

 verse the garnet porphyroblasts. 



The origin of these garnet-amphibolites which show many 

 features in common with the eclogites will be discussed on a later 

 page. 



Prehnite-Aniphibolite. 



An amphiboHte, developed as a band in the gneiss on the land- 

 ward side of Boston Island, contains prehnite as the essential 

 lime-mineral in place of plagioclase. The rock is intersected in 

 part by white venules. These, on microscopical examination, are 

 seen to consist of prehnite, associated with a little quartz present- 

 ing idiomorphic outlines. The ground-mass of the rock consists 

 of prehnite. in which are embedded hornblende and magnetite, 

 and granules of a monoclinic pvroxene. Acicular needles of 

 aetinolite are also present, and associated with quartz. 



The prehnite of the veins shows the typical radiate arrano^eraent 

 which characterizes this mineral. The refractive index is less than 

 1-61, and has a range above and below 1-63. The mean refractive 

 index is just under 1"63. The optical character is positive ; but 

 the birefrinorenee is anomalous for this mineral, and, as seen in 

 thin sections, does not exceed '019. The mineral fuses in the 

 ordinarv gas-ilame with intumescence, the fused mass being: readily 

 soluble in hydrochloric acid. The solution gives a strong reaction 

 for calcium. 



