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



II. 



101-89 



Calculating the analysis of Dr. Sipocz, we find that this rock contains 75 per cent, of 

 olivine and 25 per cent, of enstatite, which closely answers to the results obtained by the 

 microscopic examination. Thin sections, prepared from slightly decomposed specimens, 

 show that the rock is composed of a mass, often consisting almost exclusively of minute 

 grains of colourless, transparent olivine, irregular in form, and frequently not exceeding 

 0*1 mm. in diameter ; that it contains, moreover, enstatite, actinolite, and chromic iron. 



In all the slides which I have examined the grains of olivine essentially constitute 

 what may be called the ground-mass of the rock (see figs. 1, 2, and 3). They generally give 

 it, along with other elements, a microgranitoid structure. It is but very seldom that 

 the granules of peridot assume dimensions large enough to produce a microporphyritic 

 structure (fig. 2). Small sections of a light green colour scattered through the ground- 

 mass, and other transparent isotropic sections of a brownish-yellow, were found in the 

 slides. 



In the microscopical preparations, where the microporphyritic structure is observed, 

 the mineral particles which determine that structure are elliptical or irregular, but never 

 have crystallographic contours. This microporphyritic structure, however, almost always 

 passes into the banded structure, which is very decided when certain minerals, interposed 

 between the bands, assume dimensions somewhat larger than the grains of the surround- 

 ing mass. In some preparations this banded structure cannot be accurately observed, 

 especially in ordinary light, but other specimens possess it in a marked degree. The 

 explanation of this feature will serve also to throw light on some of the anomalies pre- 

 sented by the larger crystals. When specimens with this banded structure are examined 

 under low magnifying powers, they show transverse bands generally exceeding 0"1 mm. 

 in breadth (fig. 2). They are, for the most part, undulated, dark, and alternate with 

 more transparent zones. These transparent zones and dark stripes have essentially the 

 same mineralogical constitution, the difference being due, as shown by a high magnifying 

 power, to the fact that the dark bands consist of a compact mass of minute olivine grains 

 mingled with a considerable quantity of chromic iron. 



The bands, as a rule, preserve their parallelism, so that whenever there is a change of 



