WARREN. — ALKALI-GRANITES AND PORPHYRIES. 223 



shaped crystals. The grains are rounded and are to a greater or less 

 extent filled with a dusty, brownish material. It is sometimes inter- 

 grown with the quartz much after the manner described for the zircon 

 of the pegmatites. -^^ This occurrence of zircon as described appears 

 to be a characteristic of riebeckite rocks (see Murgoci, op. cit.). 

 Titanite has been already described occurring as a probable replace- 

 ment of the hornblende. It also occurs to a small extent in the form 

 of isolated grains scattered through the rock and commonly associated 

 with ilmenite or magnetite. Ilmenite or magnetite, occur as included 

 grains and more or less fine dust in all the minerals of the rock. While 

 a little of it may be primary, the greater part of it is thought to be 

 secondary even in the relatively fresh granite. Magnetite is more 

 abundant as the alteration of the hornblende has preceded further, 

 and is then noted in the form of well defined octahedra lying in or 

 about the position of the original iron-bearing silicate. Hematite 

 appears to a limited extent in the fresh granite as minute flakes or 

 grains in the feldspar and with the aegirite. In some cases its presence 

 is apparently connected with pneumatolytic processes, but in general 

 it appears only in connection with more superficial alteration. It 

 is abundant in red surface granites. 



An interesting and more unusual accessory is the mineral astro- 

 phyllite. Attention has been directed to the occurrence of this rare 

 mineral in the Quincy granite by Pirsson.^° It is only very sparingly 

 present and appears to be of irregular occurrence. It has already 

 been described as occurring about and replacing the aenigmatite and 

 the aegirite in which the latter was intergrown. It generally appears 

 in, or attached to, aegirite or the hornblende, particularly when this is 

 intergrown with aegirite. Pirsson also noted it intergrown with its 

 clea^•age direction parallel to the vertical axis of the riebeckite. The 

 writer has also seen it attached to zircon and to grains of a not fully 

 identified mineral, but one which suggested parasite in appearance. 

 Its habit and properties according to Pirsson are : minute elongated 

 laths grouped in bunches; cleavage, micaceous excellent; elongation 

 parallel to c; A = b; B = c; C = a; strongly pleochroic; A, red 

 orange, C, lemon-yellow; absorption, A>C; mean refracti^•e index 

 about, 1.7; extinction parallel to the cleavage cracks; birefringence 



19 Op. cit., p. 131. 



20 American Journal of Science, 29, p. 215 (March, 1910). 



The writer is indebted to Professor Pirsson for an opportunity to examine the 

 thin-section on which his observations were made. The mineral was more fav- 

 orably developed for study in this than in any other which the writer has seen. 



