Professor Geikie on the " Pitclosfone " of Eskdcde. 249 



from the augite, show such an affinity for that mineral as to 

 suggest that they are themselves augitic; though it is 

 remarkable to find such well-formed crystals, microliths, and 

 globulites in this conjunction. (b) In areas where the 

 globulites are most developed, the dart-shaped microliths, as 

 above mentioned, are crusted with these bodies. Owing to 

 the irregular way in which this incrustation has taken place, 

 endless variety may be noticed in the resulting club-like 

 shape of the microliths. But through them all may be 

 detected the original simple, dart-like rod, with its envelope 

 of crowded globulites. The more elongated globulites have 

 attached themselves usually by one end. (Fig. 5, PL V.) 



3. Trichites. — Under this name may be grouped a series of 

 straight, curved, or coiled hair-like opaque fibres, sometimes 

 translucent and isotropic, and then varying in tint from a 

 pale yellow, through shades of dirty green or brown, to 

 black, and in length up to -5^ of an inch or more. Some 

 of them consist of an internal pale yellow translucent hair, 

 coated with black opaque matter. These so resemble the 

 microliths that they may be only a form of these bodies. 

 Curved fibre-like microliths, not crusted with globulites but 

 sometimes coated with black opaque matter, occur in pro- 

 digious numbers in the Eskdale dyke at Kirkburn, above 

 Langholm. Their general character in mass at that place is 

 shown in fig. 6 of PL VI. The interspaces which, in the 

 more vitreous parts of the rock are occupied with pale brown 

 glass are there crowded with these microliths and with 

 globulites, and are no longer inert in polarised light, but 

 present a mottled glimmering from many points, but with- 

 out any distinct crystalline forms. This is the most complete 

 devitrification which I have met with in undecom posed parts 

 of the Eskdale dyke. It presents not a little resemblance to 

 portions of some of the intrusive basalt-rocks of Carboniferous 

 age in central Scotland. 



A remarkable devitrification of the rock is to be observed 

 in those hard '* ribs " which I have described as projecting 

 from the wall of the vitreous centre of the dyke, and forming 

 the outer envelope or crust of the vitreous columns. The 

 specimens which I have examined were all obtained from the 



VOL. V. R 



