502 THE jrARQlTp:TTE IRON-BEARINCt distuigt. 



quartz, some hornblende, aud a little epidote, with the addition in many- 

 cases of a quartz-albite mosaic between these components. 



The hornblende, as already mentioned, is a dark-green, compact A^ariety 

 that is often idiomorphic in cross-section and is often twinned. Its crys- 

 tals lie in a matrix having- the structure and composition of the aggregate 

 described above as the final alteration product of the plagioclase. In some 

 cases the aggregate is rich in biotite, while in other instances biotite is 

 present only in very small quantity, and the aggregate is practically a 

 plagioclase-quartz mosaic. 



The larger hornblendes, which appear as phenocrysts in the hand speci- 

 men, ditfer from the smaller columnar crystals mainly in size. Their rela- 

 tions to the aggregate are the same. In both cases the mineral shows its 

 secondary nature by the frayed ends of its crystals, aud by the fact that 

 large areas of almost pure hornblende are made up of bundles of small, 

 compact, columnar crystals. Another form of amphibole is frequently 

 encountered. In this many individual crystals are bound together in sheaf- 

 like bundles, with their ends extending far out into the surrounding ground- 

 mass. The rocks in which the amphiboles are of this kind resemble very 

 closely the "diorites" and altered tuffs of the Mona formation.' 



In the thin section the schistosity of these rocks is very striking. All 

 their constituents are arranged with their longer directions approximately 

 parallel, and lenses of mosaic, with their major axes running the same 

 way, wind in and out among the other components. In spite of their 

 great alteration the diabasic structure can still be detected in some of the 

 specimens, especially when their thin sections are \aewed against a white 

 background. Under the microscope this structure can rarely be recognized, 

 since the hornblendes in their growth have extended beyond the areas 

 originally occupied by the augite. Sometimes hornblende crystals, biotite, 

 and chlorite fill ophitic spaces between the decomposition products of 

 plagioclase, but these cases are rare. 



As the schistosity of the greenstones increase in amount their material 

 appears to become better crystallized, except where the foliated phases are in 

 contact with other rocks, in which case they have suffered not only extreme 



' Cf. fig. 1, PI. XVI, Ball. U. S. Geol. Survey Xo. 62, by G. H. Williams. 



