REPORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 11 



chromite of Cassin, Departement du Var, and of Baumgarten, found no transparent 

 sections of this mineral. 1 In a later note, 2 however, returning to the same subject, he 

 says that minute grains of chromite, examined with a power of 1080 diameters, are 

 translucent and even transparent, possess a reddish colour, and are sometimes even 

 decidedly red. Since the publication of Fischer's second note, Dathe 3 has shown that 

 the transparency of chromite depends neither on the minuteness of its sections, nor on the 

 high powers with which it is examined. According to him, fragments of chromite from 

 Baltimore, reduced to a coarse powder, and examined with a power of 90, and even 20, 

 diameters, were plainly seen to be both transparent and isotropic. I have compared the 

 sections observed in the slides made from the Rocks of St. Paul, and determined as 

 chromite, with grains of chromite taken from a specimen from Kisnikeff, in the 

 Oural Mountains. These grains, examined with a power of 90 diameters, show the same 

 characteristics and are perfectly identical with the yellowish-brown isotropic grains of the 

 olivine rock under description. I have found that the chromite sections are transparent 

 only when under 0*1 mm. in thickness. Triturated chromite seen under the microscope 

 very much resembles brown obsidian in its physical properties. 



After what has been advanced in the foregoing comparative analysis, I see no reason 

 for separating picotite and chromite on microscopical grounds. An examination of a 

 slide prepared from a specimen of dunite, which Professor Maskelyne had the kindness 

 to forward to me, enables me to state that the chrompicotite of that rock has essentially 

 the same characteristics as the chromite of St. Paul. The only difference I could estab- 

 lish is that in the dunite the granules of the picotite show outlines of greater regularity 

 than those of the chromite in the olivine rock. As for the presence of chrome, it is 

 easily proved by pyrognostic reactions, and Dr Sipocz has determined its percentage in 

 the analysis quoted above. 



Among the minerals playing a secondary part in the composition of the Rocks of 

 St. Paul, either because rarely present, or because constant only in some varieties, we 

 may name two, one of which can be referred to hornblende, the other to a rhombic 

 pyroxene. The sections of the amphibolic substance appear sometimes pale green 

 (fig. 3), sometimes colourless ; their contour is ill-defined, and seems to shade into 

 the ground-mass, from which, when in thin slices, they are indistinguishable, even by 

 colour. Though its colour may be less intense in some cases than in others, this mineral 

 is easily recognised in polarised light, because, when in larger and more homogeneous 

 sections, it stands out from the mass of granular olivine. 



Some of these sections perpendicular to the vertical axis are marked by a net- 



1 Zirkel, Mikroskopische Beschaffenheit, &c, p. 246 ; Rosenbusch, Mikroskopische Physiographie der Mineralien, 

 &c, vol. i. pp. 159, 160. 



2 Fischer, Kritische mikrosk. mineralogische Studien ii., Fortsetzung, 1873, p. 44. 



3 Dathe, Olivinfels, Serpentine, und Eklogite des sachsischen Granulitgebietes, Neues Jahrbuch, &c, 1876, 

 p. 247. 



