302 PROF. T. G. BONNEY AND MISS C. A. EAISIN ON [May 1 899, 



EXPLANATION OF PLATE XXIII. 



Fig. 1. X 17. Variolitic serpentine from the point north of Penrhyn-Fadog, 

 south of Four-mile Bridge and west of the Strait (p. 28(6). Ground- 

 mass of serpentine with meshwork-structure (derived from olivine). 

 Traversed by a crack filled with fibrous serpentine. 



Fig. 2. X 14. From the same slice as No. 1. A variole, embedded in serpen- 

 tine, consisting of needles of colourless actinolite, embedded in a grey 

 dustj -looking carbonate, as described in the text (p. 287.) 



Fig. 3. X 16. From the small quarry of serpentine north of Graig-fawr (p. 286). 

 A groundmass of serpentine (left blank in the figure), crossed by 

 irregular veins or bauds, mainly a carbonate, but including a very few 

 scattered fibres of (?) serpentine. The variole consists almost entirely 

 of a dolomitic carbonate, showing radial and concentric structure. 

 F'ibres of serpentine or actinolite are more abundant in the marginal 

 part, especially in some of the narrow concentric bands. 



Fig. 4. X 22. Yariolitic serpentine from the beach south of Graig-fawr (p. 281). 

 The groundmass is a pale yellowish-green, uniform-looking serpentine. 

 The abundant varioles consist externally of colourless fibres, as in 

 fig. 2, but a similar acicular mineral is more abundant in the dusty- 

 looking carbonate of the interior, and these sometimes cross at rather 

 definite angles, as is shown to some extent towards the lower boundary 

 of the drawing. 



Fig. 5. X 22. From a boulder of bastite-serpentine near the north-western 

 end of Graig-fawr (p. 288). Part of a large crystal of bastite, lustre- 

 mottled, with rounded grains. It shows a parallel fibrous structure, 

 but a confused aggregate polarization. The darker shaded part of the 

 enclosed grains is a pale green serpentine, the colourless exterior being a 

 flaky actinolite, often piercing into the serpentine ; these minerals are 

 generally parallel one with another and with the structure in the bastite. 



Fig. 6. X 16. North-east of Cerig-moelion (? a crushed banded poi'phyrite), 

 p. 296. A confusedly cryptocrystalline groundmass, the clear patches 

 being apparently a chalcedonic mosaic traversed by interrupted, roughly 

 parallel lines of ferrite. The lower part of the drawing shows the 

 edge of one of the blacker bands. The tufts are an acicular hornblende, 

 as described. 



Discussion. 



Gen. McMahon said that he had listened with great interest to 

 the lucid exposition of the paper hy Prof. Bonney, and looked 

 forward to its study when it appeared in print. An important 

 paper containing so many details could not be satisfactorily discussed 

 prior to such study. He asked whether he had correctly understood 

 the Authors to imply that the spherulitic structure of the ' vario- 

 litic' serpentines was due to secondary aqueous agents. Some 

 minerals and salts deposited from solution in water assume the 

 radiating and concentric structure of spherulites, and the fact that 

 those described by the Authors consisted in part of carbonate of 

 iron seemed to imply an aqueous origin. 



Prof. SoLLAS ventured to express his admiration of the courage 

 and success with which the Authors had attacked the difficult 

 problems involved in the study of these obscure rocks. The accom- 

 panying chemical analyses were most welcome ; the centre of 

 interest in petrology seemed likely, in the near future, to shift from 

 the microscope to the laboratory ; and evidently some reform was 



