432 



Mr. F. Rutley. 



[May 27, 



to consist in the peroxidation of the protoxide of iron in the horn- 

 blende. The general aspect of these crystallites is much darker than 

 that of their unaltered representatives, and they stand out in bold 

 contrast to the clear glass around them. 



On looking at the left-hand portion of fig. 4, Plate 3, we see a 

 number of coarse spiculae which under a lower power, as in fig. 2, 

 Plate 3, appear merely as stippling. This stippling in fig. 2 is the 

 altered condition of those parts of fig. 1 which are softly shaded, or 

 seem to be so under an amplification of 25 linear. When magnified 

 250 diameters this portion of the normal rock still appears finely 

 stippled, but contains numbers of very minute spiculae, as shown in 

 fig. 3, Plate 3. When we compare this stippling in fig. 3 with the 

 coarse spiculae on the left of fig. 4, Plate 3, the extent of the alteration 

 produced by the nine days' heating becomes striking. It is probable 

 that these spiculae are hornblende, and that they are eviut.. y a 

 further development of the much smaller ones so plentiful in the 

 glassy ground-mass of the unaltered rock. In the dusty looking parts 

 of the unaltered glass we find, under an amplification of 1150 dia- 

 meters, some indication of the source from which this wealth of 

 crystalline spiculae has been derived, fainter and smaller spiculae 

 being visible, together with sparsely distributed dark specks, a few 

 blunt-ended colourless microliths, and a profusion of colourless globu- 

 lites, as in fig. 5, Plate 3. 



The spiculae shown on the left of fig. 4 are again represented in 

 fig. 6, Plate 3, under a power of 1150 diameters. They are grouped 

 in a stellate manner and constitute a large proportion of the rock, 

 while the fine dusty matter previously visible has almost entirely dis- 

 appeared. Although, through the agency of dry heat, we have here 

 an instance of further crystalline development, yet no approach to a 

 felsitic structure is discernible in the glassy matter of this rock ; tbe 

 new crystallites which have been formed being certainly neither 

 felspar nor quartz, but possibly actinolite. 



The next specimen to be considered is a piece of black obsidian 

 from Ascension, about as typical an obsidian as it would be possible 

 to find, and showing a faintly banded structure. The general micro- 

 scopic character of this rock is shown in figs. 1 and 3, Plate 4, 

 fig. 1 being magnified 25, and fig. 3 570 diameters. In fig. 1 the 

 banded structure is well marked, and streams of colourless microliths 

 lie with their longest axes in the general direction of the banding. A 

 fragment of this rock was kept for the same period at the same 

 range of temperature adopted in the case of the Arran pitch- 

 stone. 



The rock in its normal condition is a deep black glass with a well- 

 marked conchoid^l fracture. In section, when not very thin, it 

 appears by transmitted light of a brown or coffee colour, and contains, 



