404 



F. RUTLEY OjS t THE MICROSCOPIC STRUCTURE OF 



the rock into small irregular cuboidal masses. They also describe 

 irregular circles or ellipses, the sections of spheres and ellipsoids, 

 which appear nearly to fill the spaces enclosed by the rectilinear 

 divisions. 



These straight and curved streaks have a very perceptible breadth 

 (as shown in fig. 5, PI. XXI., magnified about 12 diameters). Be- 

 tween crossed Xicols they break up into an infinity of small doubly- 

 refracting granules; and we may infer that they represent small 

 fissures which have subsequently been rilled by infiltration. 



The curved lines seem to indicate a coarse kind of perlitic or 

 spheroidal structure ; and their relation to the straight lines at once 

 calls to mind the similar phenomena, on a larger scale, in the basalts 

 of Le Puy and Rowley Regis, described by Professor Bonney*. 



In one part of the matrix there is a roundish patch, somewhat less 

 than -J^ inch in diameter, in which evident traces of perlitic struc- 

 ture are discernible. A portion of this patch, magnified 25 diameters, 

 is shown in fig. 4, PI. XXI. 



The spherules and spherulitic bands are destitute of any definite 

 internal arrangement. There is no trace either of radiating or of 

 concentric structure. Under an amplification of 575 diameters the 

 spherules are seen to consist of a confused aggregate of extremely 

 minute, colourless, rounded granules and pale green scales : the 

 latter appear to be chlorite. The little colourless granules closely 

 resemble in appearance and in dimensions the granules of spes- 

 sartine which occur in the Belgian honestones, and which have been 

 determined and described by Renardf . Owing to their extremely 

 small dimensions, they fail to occupy the entire thickness of the 

 section ; hence they are always overlain or underlain by doubly- 

 refracting matter, which precludes the possibility of ascertaining 

 whether they are isotropic. On examining a section of the coticule 

 of Dressante, near Hebronval, in Belgium, given me by Prof. Renard, 

 I find just the same difficulty, except upon the extreme margin of 

 the section, where a few of the granules have parted from the pre- 

 paration, and can be examined independently. In such cases the 

 light, of course, undergoes extinction during a complete revolution 

 between crossed Nicols. 



Returning to the Beddgelert section, a few of the minute colourless 

 granules may also be met with in an isolated condition ; and their 

 isotropic character can then be readily recognized. Under these 

 circumstances we may, perhaps, be justified in regarding them as 

 garnets, and possibly the manganese garnet spessartinei. There is, 

 indeed, a honestone, well known to the natives, and occurring in a 

 quarry at Pen-y-Gwryd at the head of the Llanberis Pass, which, 

 like its Belgian representative, contains numerous minute garnets 



* "On Columnar, Eissile, and Spheroidal Structure," Quart. Journ. Geol. 

 Soc. vol. xxxii. p. 140. 



t Memoire sur la structure et la composition mineralogique du Coticule ; 

 Brussels, 1877. 



j Blowpipe examination of the finely powdered rock shows distinctly the 

 presence of manganese. 



