270 Pirsson and Washington — Geology of Red Hill, N. H. 



Wohlerite. — The occurrence of this rare mineral for the first 

 time in an American locality is of great interest, and it has 

 therefore been reserved for special notice. As is well known 

 it is a mineral characteristic of the nephelite-syenite pegmatite 

 dikes of South Norway and has been made the subject of a 

 special study by Brogger.* At Red Hill it is not found every- 

 where distributed through the rock mass, but appears to be 

 confined to those places where the increasing size of grain or 

 the arrangement and characters of the minerals suggests that 

 they have crystallized under the influence of mineralizing 

 vapors which were there especially active. The specimens 

 which mostly contained it were not taken from the quarry 

 which afforded the type material but from blocks in the stone 

 wall near it. Under the heading of "texture" it is mentioned 

 that one type in particular contains the mineral. 



Q In the hand-specimen it is seen as minute 



honey -yellow, resinous-looking masses or par- 

 ticles, averaging about O^™" 1 in diameter. 

 They are commonly associated with the 

 hornblende though sometimes wedged 

 among the feldspathic minerals. The gen- 

 eral appearance in fact is much like that 

 of minute titanite crystals. They are too 

 small and irregular or broken to present 

 definite crystal outlines or faces. Under the 

 microscope at first glance they recall large 

 apatite crystals in their general form, rela- 

 tive abundance and tendency to be grouped 

 in and about the ferromagnesian minerals. 

 They have been observed partly embedded 

 in hornblende, and on the other hand, one 

 was seen containing a zircon ; therefore 

 f Red b^^n^h 6 their period of crystallization was last among 

 the accessory group, such as apatite, zircon, 

 etc., and previous to the completion of the crystallization of 

 the ferromagnesian group. 



Several of the crystals were fortunately cut nearly parallel 

 to the face 010 and perpendicular to the obtuse bisectrix c; this 

 is the most characteristic section through the mineral and per- 

 mits of its ready identification. The appearance of these crys- 

 tals is shown in the adjoining figure (fig. 3). They were 

 found to have the following characters. Colorless or very 

 faint, scarcely perceptible, yellow and nonpleochroic. Relief 

 rather high, a little above apatite; refractive index about 1*7. 

 Birefringence in the section 0*011 ; highest observed in others, 

 - 025. A bisectrix c emerges almost perpendicular to the sec- 



* Zeitschr. fur Kryst, vol. xvi, p. 351, 1890. 



