ON A BOULDER OF HORNBLENDE PICRITE. 



137 



11. On a Boulder of Hornblende Picrite near Pen-y-Carnisiog, 

 Anglesey. By Prof. T. G. Bonnet, M.A., F.R.S., Sec. G. S. 

 (Bead January 5, 1881.) 



Last summer, through the kindness of Professor Rosenbuseh, I was 

 able to examine several specimens of picrite in - the Geological 

 Museum at Heidelberg, and to study the rock in the field, near the 

 village of Schriesheim, a few miles to the north of that town. In 

 September I was walking with some students along the road which 

 leads out of the village of Pen-y-Carnisiog northwards to Bwlyn 

 (Anglesey), when I observed, in a field on the left, the fractured 

 face of a boulder, in which a number of large crystals resembling 

 augite, glittering in the sunlight, in a dull dark matrix, recalled at 

 once the characteristic aspect of the Schriesheim picrite. The 

 boulder had been broken, apparently rather recently, into three 

 pieces, one much smaller than the other two ; and its volume must 

 have been not much less than a cubic yard. In its weathered sur- 

 face and toughness under the hammer it also resembled the Schries- 

 heim rock. In both, the larger crystals (which are often about 

 two thirds of an inch long) contain a number of dark serpentinous- 

 looking enclosures, giving to the cleavage-faces an interrupted lustre 

 somewhat resembling (except in the absence of a metallic gleam) 

 that of bastite. The Pen-y-Carnisiog rock looks a little more de- 

 composed ; but macroscopically the resemblance between my two 

 specimens is so great that one could believe them to have been 

 broken from different parts of the same mass. 



Part of a Slice from a Boulder of Hornblende Picrite near Pen-y- 

 Carnisiog, Anglesey. (Magnified 30 diameters,) 



a. One of the grains of altered olivine. 



b, b, b. Aggregated small crystals of hornblende, probably of secondary origin. 



