504 Pirsson and Washington — Geology of New Hampshire. 



developed along cracks. The grains of titanite in such cases 

 often have a nearly parallel orientation. Sometimes the out- 

 line of the pseudomorph is well defined by grains of titanite 

 running along the boundary and more or less contiguous ; the 

 interior is mostly chlorite with scattered areas of the titanite. 

 The amount of calcite present is usually relatively small and 

 very often entirely wanting. The whole mode of occurrence 

 of the titanite shows clearly that it is a secondary mineral ; 

 that it is quite similar to that of epidote in other cases of horn- 

 blende alteration. That the mineral is titanite, however, and 

 not epidote is very clear, its refractive index and birefringence 

 are far too high, and in sections perpendicular to the acute 

 positive bisectrix it shows the characteristic optical figure — the 

 small angle of the optic axes with both hyperbolas in the field 

 with the strong dispersion of the optic axes which causes them 

 to be bordered red and blue, with red greater than blue. 



It might also be suggested that there was an original inter- 

 growth of titanite and hornblende, but this is negatived by the 

 fact that the unaltered fragments and crystals of hornblende 

 are clear and free from inclusions ; only where it is altered 

 does the titanite appear. 



In considering the cause of this manner of alteration, it 

 should be remembered that basaltic hornblendes have been 

 shown by Schneider* to have a pretty constant composition 

 and to contain Ti0 2 in amounts ranging from 4*26 to 5*40 per 

 cent in the specimens analyzed by him, averaging about 5 per 

 cent. Brogger,f in his table of the alkali-iron hornblendes, 

 calls attention to the fact that these hornblendes rich in TiO a 

 have extinction angles of 0-10° and pleochroism brown to 

 light yellow, c > b > a, characters like those of the one under 

 consideration. 



It may be considered certain that this hornblende contains 

 several per cent of Ti0 2 and that in the breaking up of its 

 molecule through processes of alteration this oxide has united 

 with the lime and silica to form titanite. Five per cent of 

 TiO, would yield over 12 per cent of titanite ; the amount 

 present appears considerably greater than this, and moreover 

 it is not confined to the hornblende pseudomorphs but also 

 occurs scattered through the groundmass in the same irregular 

 grains. The larger grains of iron ore, when examined with a 

 high power, are not solid mineral as is usually the case, but are 

 filled with a network of holes, so that they are mere sponges. 

 This suggests another source for the Ti0 2 , as that wander- 

 ing out from the iron ore could have aided in a further 

 increase of the titanite already forming from the hornblendes, 



*Zeitschr. fur Kryst. xviii, 580, 1890. 



f Grorudit-Tinguait Serie, Vid-Selsk. Skrifter, M-N-KL, 1894, No. 4, p. 22. 



