246 Proceedings of the Royal Ehysical Society. 



assume the form of irregular crystalline granules. The mag- 

 netite occurs in abundant and tolerably well defined crystals. 

 No olivine was observed. These minerals are dispersed 

 through a base relatively small in quantity, and with distinct 

 action on polarised light. This substance is crowded with 

 minute clear microliths, which are often to be observed 

 specially abundant round the augitic ingredients. There 

 occur also curved, club-shaped, opaque microliths, which 

 occasionally radiate in a tufted manner from a common 

 centre. The rock has undergone some alteration, as is shown 

 by filaments of calcite and a good deal of diffused " viridite " 

 and "opacite." A specimen taken from Wat Carrick Hill, 

 rather fresher in condition, shows a large proportion of 

 base of a pale brown tint, dusty or granular, and crowded 

 with fine curved dark-brown to black hairs and straight 

 yellow rod-like microliths. The felspar contains in some 

 of its prisms abundant yellow granules and rods which are 

 probably augite. The augite is well crystallised, but its 

 prisms are remarkably small. The magnetite occupies a less 

 prominent place in the rock, both as to quantity and size of 

 individuals, than at Shaw Burn, its octohedra ranging from 

 -gig- of an inch to exceedingly minute form. 



(2.) Microscopic Structure of the Central Vitreous Band. 



The vitreous central band of the Eskdale dyke, or Jameson's 

 " pitchstone," is full of interest when studied with the micro- 

 scope. Examined with even a low magnifying power it is 

 readily seen to consist of a clear glass through which are 

 diffused abundant crystals of striated felspar, augite, magne- 

 tite, and many small microliths. 



The Glass. — The abundance of this substance at once marks 

 a strong contrast between the central band and the marginal 

 zones of dolerite. Though black and opaque in thick fragments, 

 the glass in extremely thin sections appears pale brown or 

 almost colourless, and perfectly transparent. For the most 

 part it is remarkably homogeneous, and remains absolutely 

 inert in polarised light. But it is marked by the occurrence 

 of clots and streaks of a rich dark-brown tint, which are also 

 quite unindividualised. The clot-like patches, which may 



