1909-1910.] 3 I 7 



When examined closely the veins were found to have a 

 composite structure. Close to the matrix was found a layer of 

 chlorite, varying in thickness from a mere film to rather more than 

 i cm. Upon this was deposited a layer of varying thickness, 

 composed of the carbonates of calcium and magnesium. Finally, 

 in the interior of the vein was found the chalcedony, which passed 

 occasionally into common crystallised quartz. A peculiar feature 

 was the intergrowth of the carbonates with the chalcedony, 

 producing a pure white opaque substance, which might be truly 

 regarded as a white jasper. In passing from the blue basalt to 

 the purple rock touching the veins some noteworthy features were 

 observed microscopically. A slice of the rock intermediate in 

 position between the two revealed the presence of an orange-red 

 serpentine contiguous with the chlorite, both in the cracks of the 

 olivine and the interstices of the rock. The percentage of 

 magnetite was also found to be largely increased. In the purple 

 rock the olivines were observed to be completely replaced by 

 chloritic and serpentinous minerals of blue-green and orange-red 

 colours respectively ; the mixture of these colours gave rise to the 

 purple colour. The magnetite in the purple rock was found to 

 have increased still more, and the augites were of increased size 

 and showed a distinct tendency to ophitic structure with regard to 

 the felspar. Chemical analyses shewed that as the veins were 

 approached the basalt increased in (i) silica, and (2) in iron. 

 The relative proportions of the magnetite might be roughly 

 represented by the following figures: — Blue basalt, 1. Inter- 

 mediate basalt, 1.5. Purple basalt, 2 to 3. 



In conclusion, Mr. Strachan conpared these structures with 

 similar structures in other local rocks, and gave reasons for 

 believing that the vein-stuffs of the Ballyboland basalt were not 

 the product of solfataric alteration, but that the purple rock and 

 the other minerals found in the veins were formations due to 

 magmatic separations during the cooling of the molten rock. The 

 relative position of the minerals was strictly in accordance with 



