IGNEOUS ROCKS 465 



be locally very abundant. While sometimes colorless, it is often of a deep 

 purple color. It forms shapeless grains or aggregates and is commonly 

 found in close association with the riebeckite and ffigirite. It is also com- 

 monly found associated with a finely granular material whose exact char- 

 act-er is somewhat in doubt. This material forms masses of minute 

 rounded or irregular grains which rarely reach a diameter of 0.2 milli- 

 meter. They are colorless to pale yellow, are usually filled with dusty 

 particles, possess a high index of refraction, a strong double refraction, 

 and not infrequently an obscure fibrous, radiated structure. In reflected 

 light they have the peculiar white color generally associated with leu- 

 coxenic material. This granular material is very closely associated with 

 the asgirite and riebeckite, particularly along zones in which shearing 

 seems to have been particularly strong. The grains appear sometimes to 

 be a replacement of segirite or perhaps of the riebeckite which occurs with 

 it. In some instances black oxide particles are found in the granules. 

 Though the properties described for this material are not very positive, 

 it is thought that it is in the nature of a leucoxenic alterations^ developed 

 during the period of shearing, and is also very likely closely connected 

 with pneumatolitic action prevailing in the granite at that time. In fact, 

 the general textural appearance of the granite is such as to suggest that 

 the shearing may have accompanied the intrusion and consolidation of 

 the rock when mineralizers were still active, and that the granular ma- 

 terial described, as well as the astrophyllite, were a product of this period 

 which also affected the textural relations of the riebeckite, segirite, and 

 quartz. 



Quantitative studies. — It has not been possible for the writers to make 

 chemical analyses of the riebeckite rocks. However, a Eosiwal estimate of 

 the. mineral composition has been carried out on a large thin-section cut 

 from a typical specimen of granite from one of the quarries. Although 

 not as precise as could be desired, on account of the textural peculiarities 

 of the rock, it will serve to show very well the approximate mineral com- 

 position, and from it has been calculated the approximate chemical com- 

 position. These are given below, together with a chemical analysis of the 

 "riebeckite porphyry" ^^ from the same mass, of the Quincy granite, and 

 of the riebeckite granite from the Island of Sokotra. 



^5 This identification is supported by the occurrence of titanite as an alteration product 

 of the riebeckite in the Quincy granite (loc. cit., p. 217). 



36 The specimen on which this analysis was made is said by Enaerson and Perry to 

 have come from the "top of hill one mile northeast of Sneech Pond," which according to 

 their map would seem to be in their "granite-porphyry" area. From their description 

 and from the writers' acquaintance with the field, it would appear to be a granite 

 porphyry representing (see later) a phase of the igneous mass occurring nearer the 

 porphyry than the granite of the quarry whose composition has been estimated by us. 



