MUSEUM OF COMPAIUTIYE ZOOLOGY. 



241 



glomerate (described with section [2] ) ; though what seemed to be the 

 remains of olivine crystalis were found in one or two other sections. 

 Their relations to the groundmass prove an anterior origin : some of the 

 magnetite and opacite in the sections have probably been derived from 

 the alteration of small fragments of olivine. 



Between the lafgc and small feldspar crystals lies a mass of greenish 

 alteration products, — chlorite (often dichroic), viridite, magnetite, opa- 

 cite, considerable cpidoto, quartz, and calcite. When some of the large 

 feldspar crystals diverge, the triangular space between them is filled 

 with very small feldspar crystals, lying in this greenish mass ; showing, 

 as has been often remarked, that it is merely an original, glassy base, 

 much altered, for we find this same relation in the unaltered basalts. 

 Calcite, quartz, epidote, hornblende, biotite, apatite, etc., in |;he decom- 

 posed base, seem to belong to the more advanced state of decomposition. 



Magnetite is always present. A large part of the magnetite arises 

 from the decomposition of the base, and it is generally difiicult to say 

 what part of it is original. 



While in some sections true amygdules are wanting, yet they gener- 

 ally occur, characterized by their sharp boundary, and the arrangement 

 of the feldspars of the groundmass parallel to their outline. They are 

 filled by epidote, chlorite, viridite, calcite, or quartz; the epidote gener- 

 ally on the outside, when other minerals occur with it. Besides these true 

 amygdules, ai'cas of decomposition occur in the groimdmass, consistiuf*- 

 either of opaque ferritic material, constituting the macroscopical red 

 spots, or of epidote, chlorite, viridite, etc., enclosing the small feldspars. 



Assuming that all the specimens described belong to the same rock- 

 mass, this rock, according to the classification used, would be referred 

 to both the Diabase and Melaphyr sections of the Basalts* (according to 

 the specimen examined), or again might be called a Diabase and Olivine- 

 diabase.t It is found by study to be a rock which, in the original state, 

 was composed of the feldspars, olivine, magnetite, a base (glassy, micro- 

 lithic, etc.), and probably some augite (though this cannot be identified 

 now), all in varying proportions, and that these original constituents 

 have been largely replaced by secondary products. It is therefore an 

 altered basalt, as has been previously shown by others for similai- rocks 

 of this region. J 



* M. E. Wadsworth "On the Classification of Rocks," Bulletin Mus. Comp. Zool., 

 Vol. y. No. 13. 



t Rosenbusch, Mikros. Phys. der Mass. Gest., etc. 



.t M. E. Wadsworth, Proc. Bost. Soc. I^at. Hist, Vol. XIX, p. 217 et scq., 1877. 

 E. R. Benton, Ibid., Vol. XX. p. 41G et seq., 1880. 

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