28 TRANSACTIONS OF THE [oCT. 19, 



border to an almost complete replacement of the rutile. The 

 mass of titanite, which results from this alteration, is not made 

 up of a single crj'stal as is the case when it occurs as an original 

 constituent, but is composed of minute grains closel}^ compacted 

 together. 



Riess and Zirkel mention olivine as being present in some of 

 the rocks, but the writer was unable to identif}'^ it in any of the 

 slides, although they represented the different localities where 

 it was said to occur and special attention was directed upon 

 finding it. 



It would seem, however, that Riess has confused it with 

 titanite. In his description of its properties, he says : " In all 

 the above hornblende-garnet rocks olivine is present in large 

 quantities. The olivine occurs as dirty white grains. The ap- 

 pearance of these grains in the form of opaque particles crowded 

 together, or of a zircon crystal lying in the middle of the 

 grain is frequent. The chromatic polarization of the olivine 

 is weak." While the optical properties of the two minerals ai-e 

 quite distinct, the forms which the titanite or titanomorphite 

 assumes, often bear a rude resemblance to those of olivine, and 

 this fixct ma}' have lead to the mistake in identif3ing them. 



Quartz was found in some of the slides and was absent from 

 others. While often an original constituent, in some cases its 

 presence is due to later infiltration. 



Feldspar is seldom if ever found in the omphacite rocks, but 

 accompanies hornblende and may then occur in considerable 

 quantities. It is a plagioclase whose extinction angles indicate 

 andesine. 



Other less common minerals are muscovite, pyrite, p3'rrhotite, 

 magnetite, apatite, calcite and zircon. 



Macroscopically the eclogite has generally a massive appear- 

 ance. Though occasionally a well-defined parallel arrangement 

 of the different minerals can be detected, it never assumes the 

 schistosity of the surrounding gneisses. 



A peculiar structure which many of the slides exhibit arises 

 from the intergrowing of pyroxene, hornblende and feldspar 

 giving an effect not unlike that to be seen in the granophyric in- 

 tergrowth of quartz and feldspar in the acidic eruptive rocks. 

 Oftentimes their more or less irregular arrangement of the grains 

 gives way to a radial arrangement about some pyroxene or gar- 

 net crystal. The latter is then generally surrounded by a dense, 

 grayish coloi'ed rim, not unlike kelyphite in its appearance, 

 which under high powers resolves itself into an aggregate of 

 feldspar, hornblende and omphacite. 



The chemical composition of the eclogite is somewhat vari- 



