482 F. KTJTLEY ON SOME PECULIARITIES IN THE 



Pig. 4, Plate XXIII. is the drawing of a crystal selected from many 

 others of a similar nature. These crystals occur, mostly in radiate 

 or fan-shaped aggregates, in a dark spherulitic-looking obsidian 

 from Mexico. The specimen was given me by Mr. H. W. Eristow. 

 In outward aspect it resembles a very bad sample of bottle-glass, 

 but under the microscope is one of the most beautiful and intensely 

 interesting specimens that I have ever seen. With the exception of 

 the crystal aggregates just alluded to, the glass is wonderfully free 

 from any impurities, containing only clear spherical bubbles, which 

 by refraction show numerous sharply denned concentric rings. The 

 crystals present the appearance of being either partially developed 

 or partially disintegrated, either as though the process of crystal- 

 lization had been rapid and suddenly arrested, or as though the 

 crystal had been partially dissolved and the process suddenly stopped, 

 leaving a beautifully dissected structure, the very framework appa- 

 rently of the crystal. In the instance here figured, and which is 

 represented as seen by dark-ground illumination, the framework 

 consists of two well-defined arcs or, rather, hyperbolic curves re- 

 sembling those in the example just described, which span the crystal 

 longitudinally, their concave aspects girding the exterior lateral 

 portions of the crystal. These arcs are quite distinct and separate ; 

 they are thickest in the middle, tapering gradually towards their 

 ends, which reach the extreme corners of the crystal. Prom each 

 of the four cornua processes, which, I believe, are pectinate in cha- 

 racter, are given off on either side of the main rib in directions 

 which apparently correspond with the direction of the corresponding- 

 portion of the adjacent main rib ; hence they do not form right 

 angles with the limb from which they are developed. They have a 

 delicate feathery appearance, and disappear towards the thicker, 

 median portion of each arc in this particular crystal. The dark 

 patch in the corner of the drawing represents the glassy magma ; 

 the light portion indicates part of a large underlying crystal which 

 is out of focus. We see in the framework of this crystal the same 

 large divisional lines which mark off the striated and, at times, un- 

 striated areas in the sanidine crystal from the Berkum trachyte. 

 We see also in this instance either that development of strise com- 

 mences at the angles, or that disintegration begins midway along 

 the sides. Pig. 1, Plate XXIV. shows a very minute, I might say 

 embryonic, crystal occurring in this Mexican obsidian, magnified 240 

 diameters; unfortunately it is mounted under a glass cover which is 

 too thick to permit the use of a higher power* We here see a 

 straight stem which gives off straight processes on either side. These 

 processes run at an oblique angle to the main stem, the one set 

 forming with the other set an angle of 115°, as nearly as I could 

 measure it under so low a magnifying power. Other processes are 

 given off from them, which we will, for convenience, call secondary 

 processes ; these may pass from either side of the primary processes, 

 and appear to follow two directions, one of which is parallel to the 

 main stem, while the other is parallel to the opposite series of pri- 

 mary processes ; so that these secondary processes also make an 



