Reviews — Dr. J. Petersen on Cheviot Rocks. 227 



surprised to find that it contains work of tlie very highest order. 

 As the rocks have already been described to some extent by myself 

 in the pages of the Geological Magazine, it will probably interest 

 readers to know how far the observations of Dr. Petersen tend to 

 confirm my own, and how far they are opposed to them. It may be 

 as well to state at the outset that the rock which I first recognized 

 as belonging to the group of the so-called augite-andesites, and 

 subsequently ventured to call hypersthene-andesite, when Prof. 

 Eosenbusch had taught me how to recognize the rhombic j^yroxene, 

 is denominated by the author enstatite-porphyrite. The term enstatite 

 being selected as the representative of the entire group of rhombic 

 pyroxenes, and the term porphyrite being given to the rock because 

 it is of Pre-Tertiary age. 



The fresh rock ("pitchstone porphyrite" of the older authors) is 

 described as follows. In a black, or brownish-black ground-mass, 

 having a pitchy or greasy lustre, lie glassy felspars, with sharp 

 boundaries, which are usually elongated in the direction of the a-axis 

 and which often show twin-striation, A pyroxenic mineral is not 

 clearly recognizable by mauroscopic examination. Narrow brick-red 

 veins traverse all the specimens examined by the author. Thin 

 sections of this rock show under the microscope hematite, magnetite, 

 apatite, pja-oxene, felspar, and a glassy base. The pyroxene, which is 

 by far the most interesting constituent, belongs to two crystal systems, 

 the monoclinic and rhombic ; the latter form largely predominating.^ 



The rhombic pyroxene is described in detail, but it will only be 

 necessary here to call attention to certain points not referred to by 

 myself. The author observed in many cases brown or black leaf or 

 rod-like interpositions which are arranged parallel to the vertical 

 axis. He describes also a well-marked pinacoidal cleavage, which I 

 v/as not able to make out. The pleochi'oism referred to the crystal- 

 lographic axes is thus described : rays vibrating parallel to the c, h, 

 and a axes are green, yellow, and reddish-yellow respectively. 

 The polarization tints of the rhombic pyroxene are dull, whereas 

 those of the monoclinic mineral are bright. The author agrees with 

 Bricke in regarding this as an important means of distinguishing 

 between the two pyroxenes. 



After separating the two pyroxenes from the other constituents 

 by means of the Sonstadt solution, the author succeeded in obtaining 

 the rhombic mineral in a state of great purity by means of the 

 solution of borotungstate of cadmium. It was found that in a 

 concentrated solution of the latter salt both varieties swam, but that 

 by slight dilution the rhombic mineral was caused to fall whilst the 

 other floated. One gramme of the mineral thus isolated, having a 

 specific gravity of 3-331, gave the following analysis. I add my 

 own analysis by way of comparison : — 



1 It seems hardly necessary to give additional evidence of the distinctness of these 

 two pyroxenes. I may mention, however, that 1 have recently mounted a specimen 

 of the monoclinic mineral, so that I can rotate it round the (;-axis under the 

 microscope. It is green in colour, and does not show a trace of pleochroism in any 

 position. 



