SoLLAS — On the CrystnlUne Form of Rieheckite. 521 



produced on cooling through 100° C. could not give rise to a deviation in 

 any of the angles of the cube of more than a small fraction of a miaute. 

 The effect is greater if quartz be supposed substituted for felspar, but 

 still quite inadequate. If instead of considering merely the mean 

 coefficient of expansion of these minerals we take into account the 

 coefficients of expansion in different directions a more marked 

 result would, in certain cases, follow ; but this would be dependent 

 on a definite orientation of material which could not legitimately be 

 predicated. Strains due to unequal contraction will thus scarcely 

 help us, and the complex growth of the crystals may be admitted to 

 furnish by itself a sufficient explanation, 



Brogger,^ in studying Arfvedsonite, encountered similar anomalies to 

 those we have described, and explains them in a similar way. " Nach 

 der Mehrzahl der ausgefuhrten Messungen zu schliessen, wiirde der 

 Arfvedsonit dem asymmetrischen Systeme angehoren ; obwohl aber 

 die einzelnen Messungen fiir sich in den meisten Fallen recht voU- 

 kommen schienen, diirfte eine derartige Schlussfolgerung doch gewiss 

 nicht berechtigt sein, derm die erwahnte, Ubrigens gewohnlich ver- 

 steckte, subparallele polysynthetische Aggregation mehrerer Individuen 

 erklart geniigend die beobachteten Abweichungen von der monosym- 

 metrischen Symmetrie." 



Such polysynthetic aggregates would seem to offer a link between 

 homogeneous crystals and radiate fibrous growth. 



"While this communication was passing through the press, I re- 

 ceived, by the kindness of the author, an important Memoir by 

 Professor Brogger,^ in which appears (p. 33) a review of those species 

 of hornblende characterised by containing iron and soda. Professor 

 Brogger has shown that they fall into a series in which an increase in 

 the angle of extinction (c : C) is correlated with an increase in the 

 contents of iron and soda. The series is as follows : — 





Angle of Extinction. 



Basaltic hornblende from Bohemia, 



0°-10° 



Barkevite, 



14° 



Hornblende of Solvsbergite, . 



31°-34° 



Hornblende of Grorudite, 



. 41°-58° 



Arfvedsonite, .... 



76° 



Rieheckite (?), 



86° 



1 Brogger, Zeits. f. Krystallographie u. Min. xvi., p. 403, 1890. 



2 "DieEruptivgesteinedesKristianiagebietes." i. Die Gesteine der Grorudite- 

 Tinguait-Serie vonVV. C. Brogger. " Videnskabsselskabets Skrifter." i. Mathe- 

 matisk naturv. Elasse, 1894, No. 4. 



