Pirsson and Washington — Geology of New Hampshire. 505 



which would supply the necessary lime and silica. The basal- 

 tic hornblendes contain about 12 per cent of lime, and if this 

 were all used in producing titanite it would make 42 per cent 

 in weight of the original hornblende. But as some of the 

 lime has gone into calcite the amount of the mineral replac- 

 ing hornblende is not so large as this, but is between this 

 figure and that given above — probably 20-30 per cent and 

 varying in different cases. 



The two analyses of camptonose rocks previously given 

 show about 4 per cent of Ti0 3 in the rock, and as this has 

 apparently all gone to form titanite, the total amount present 

 is about 10 per cent in weight of the rock mass. 



The appearance of the rock, and the white mica in the feld- 

 spars would seem to indicate that the alteration was due to 

 hydrothermal metamorphism rather than to atmospheric 

 weathering. 



The occurrence of titanite as a secondary mineral seems not 

 to have received the attention that it deserves. Every one is 

 familar with its appearance in mantles around titanic ore 

 grains in gabbroid rocks, and it is often mentioned in such cir- 

 cumstances as of secondary origin, but we are not acquainted 

 with any description of its derivation from another mineral 

 such as hornblende, though this may of course have been 

 mentioned in the literature. In this connection it is interest- 

 ing to recall the fact that it often appears in chlorite schists, 

 amphibole chlorite schists and amphibolites whose chemical 

 compositions are similar to those of magmas which yield salf emic 

 and dofemic rocks. 



Gunstock Gneiss. 



As stated in the geological portion of this paper, the igneous 

 rocks of the Belknap massif are in contact with micaceous 

 gneisses along the western boundary. They constitute a dis- 

 tinct formation worthy of especial study, but as they are 

 heavily covered with drift and exposures are none too frequent 

 this would have taken more time than was possible to devote 

 to this purpose and would have led us away from the main 

 subject of this study. In our work along the western contact, 

 however, we came upon these rocks in a number of places and 

 specimens from several of them were taken for investigation. 

 From two of these, one from the borders of the little ravine 

 on the southwest foot of Locke's Hill and the other in the pas- 

 ture fields at the foot of the steep slopes on the west side of 

 Mt. Gunstock, sections were cut and studied and the results 

 are given beyond. Since the lower valley of the Gunstock 

 River is cut in this gneiss, we may provisionally, for purposes 

 of reference, term it the Gunstock Gneiss. 



