1835.] Porphyritic G^reenstone Rocks. 281 



lies often in single, regularly defined crystals in the labra- 

 dorite. 



The hypersthene rock of the Isle of Skye, is well 

 characterized. The Royal collection contains excellent 

 specimens, procured by H. H. Dechen and Oeynhausen. 

 The rock is very coarsely granular ; the quantity of hypers- 

 thene almost predominates, which has never the common 

 brown, but a blackish-green colour; the labradorite is 

 greenish-white, and translucent. Titaniate of iron occurs 

 in it, but in small quantity. 



In the Hartz the hypersthene rock is very frequent, and 

 forms the greatest portion of the greenstone which occurs 

 there : Yet the different varieties can be scarcely discrimi- 

 nated, even in the most distinct specimens ; the labradorite 

 is opaque and greenish-white; the hypersthene brown. 

 Larger crystals of labradorite lie frequently in the small 

 granular mass, which also contains titaniate of iron and iron 

 pyrites. To these distinct varieties belong the hypersthene 

 rock of the Petersklippe, in the vicinity of Buchenberg, 

 near Wernigerod ; of Heinrichsburg, near M'agdesprung, 

 in Selkethale; of Huththale, near Clausthal ; and of KoUie, 

 near Braunlahe (Nos. 63 and 64 of Lasius' Collection of 

 Hartz Minerals). These species of rocks resemble those of 

 Krotenmiihle, near Steben, in the Fichtelgebirge. 

 CTo be continued. J 



Article V. 



On the Phenomena of Accidental Colours. By M. Plateau. 

 C Abridged from the Annales de Chimie et de Physique, 

 tome. 53, p. 386. By Charles Tomlinson, Esq.)^'^ 



[1 .] *' We must regard, as entirely unsatisfactory, the theory 

 of accidental colours most generally admitted, namely, that 



* Although the Editor, in the last Number of this Journal, has given an epitome 

 of this theory sufficient to convey a general idea of its nature, yet, when a theory 

 is to be discussed and examined, it is necessary the Author should be heard. But, 

 lest it may be thought by some, that too many pages in the present Number are 

 devoted to this subject, I have been induced to divide the translation of this paper 

 into two parts. The first part, now inserted, is a necessary accompaniment to my 

 own paper. The second part will be published next month, when Ihope to be 

 able, time and health permitting, to continue the investigation*— C. T. 



