192 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



of iron oxide, hydrous and anhydrous, which constitute the pigment 

 in the rock. 



Dior it c (Cat. No. 75,486). — As with the diorites already described 

 (see p. 187), so here also is the orientation of the essential constitu- 

 ents, hornblende and feldspar, such as to produce a decided sheared 

 or schistose structure. Besides these two constituents, an occa- 

 sional speck of light brownish-gray silvery mica may be seen, usually 

 included in the hornblende. 



Hornblende. — This mineral is present in irregular and connected 

 grains, strongly pleochroic, with vibrations of the following color 

 scheme : C, generally, bluish-green ; B, green, and A, yellow with 

 absorption B> O A, with B and C lying very close together. 

 Of inclusions, few were noted, an occasional irregularly shaped 

 magnetite, a fragment of feldspar, and an occasional speck of brown 

 mica, constituting the list. 



Upon closer examination the hornblende presents many points of 

 rather unusual interest. Its color has been described as green; 

 this statement, however, needs qualification, for in spots the mineral 

 is perfectly colorless, other amphibolic characteristics remaining in 

 full perfection. This bleaching of the mass of the crystal is appar- 

 ently not connected with decomposition or alteration in any way. 

 The cleavage lines of the mineral stand out full and clear. The ex- 

 tinction angles remain as in the contiguous green portions and the 

 mineral extinguishes as a unit. The interference tints of the 

 bleached portion are higher than those of the green parts. A 

 bleaching, similar to this, has been remarked by G. H. Williams^ 

 in the hornblende of cortlandite. This author says : " The mineral 

 (hornblende) becomes colorless and consequently non-pleochroic, 

 while retaining the compact structure and optical behavior of the 

 unaltered portion ; later there is developed, particularly around the 

 edges of the hornblende, a bright, emerald green substance which, 

 on account of its lack of dichroism and feeble action on polarized 

 light, may be regarded as chlorite." The hornblende of this speci- 

 men also exhibits a uniform bleaching in its peripheral portions, and 

 the hornblende has doubtless become chlorite in these places. Very 

 often the spaces, or boundaries between adjacent hornblendes, are 

 filled with sericitic material which has come from the feldspars. It 

 is possible that the waters which have produced this sericitic change 

 may have bleached the hornblende along its path, thereby changing 

 its composition to the less ferruginous chlorite. 



^ Am. Jour. Sci., xxxi, 1886, p. 34. 



