S. M. Bernard — Developing Trilohites. 553 



the twin-line is 5°. These observations agree precisely with Michel 

 Levy's results for oligoolase of composition Ah^ An-^. The green 

 mineral in this rock forms distinct slender rods and skeleton-crystals 

 about 0-03 inch long, with occasionally a stouter crystal or pseudo- 

 morph. Most of it seems to be hornblende, but some is a feebly 

 polarising chloritoid substance : possibly both represent an altered 

 pyroxene. The specific gravity of this rock is 2-663. 



Another specimen, richer in the green mineral [1757], gave 

 2-763, and yielded 59 8 per cent, of silica.^ Oligoclase of com- 

 position Ah^ An^ has a specific gravity 2-659 (Tschermak) or less 

 (Goldschmidt), and contains 61-9 per cent, of silica: the higher 

 density and lower acidity of our rock is sufficiently accounted for 

 by the considerable quantity of a ferro-magnesian mineral which it 

 contains. A similar mineral occurs more richly in a six-inch dyke 

 traversing the gabbro of Iron Crags [1881], and this rock has a 

 specific gravity of 2-906. Here, however, the little felspar-fibres 

 have lost their radiate arrangement. 



Eeaders familiar with British and foreign " variolites" will recog- 

 nise in the preceding short descriptions features characteristic of 

 those curious rocks, or perhaps more accurately features often found 

 in intimate association with the more typical variolitic structures. 

 But while such structures have usually been described in distinctly 

 basic rocks, we have them here reproduced in rocks of intermediate 

 chemical composition. Other veins and dykes in the neighbourhood 

 are of thoroughly basic rocks, and if, as seems probable, the whole 

 group had a common origin, we have here another problem in the 

 differentiation of rock-magmas in this very interesting district. 



V. — On the Application of the Sand-blast fok the Develop- 

 ment OF Trilobites. 



By H. M. Bernard, M.A., Cantab., F.L.S., F.Z.S. 



MY endeavours to establish a genetic relationship between the 

 Annelida and the Phyllopod Crustacean Apus^ led me, very 

 naturally, to the study of the Trilobites. Specimens were kindly 

 lent me by Dr. Woodward, F.R.S., Prof. Judd, F.R.S., and by Prof. 

 G. B. Howes, it being my privilege at the time to be working in the 

 Huxley Eesearch Laboratory, which is under his direction. 



Picking at the fossil with a steel point taught me, what every 

 worker soon finds out for himself, that the limbs which, under such 

 a dorsal shield, would probably have been thin-walled structures, 

 could not be differentiated from the matrix by any such method. 

 Was there, then, no mechanical method of revealing the under- 

 surface? At first I thought, among other things, of a stream of 

 water, mixed with emery powder, forced through a rose ; this idea 



1 This was kindly determined for me in the laboratory of the Yorkshire College of 

 Science under the direction of Dr. J. B. Cohen. 



2 "The Apodidaa," Natiu'e Series, 1892, and the Systematic Position of the 

 Trilobites, Quart. Journ. Geol. Soc. August, 1894, Yol. L. pp. 411-432. 



