Professor Geikie on the " Fitchstonc " of Eskdale. 245 



for crystallisation. The marginal portion, therefore, repre- 

 sents more nearly than the central part what was the condi- 

 tion of the rock at the time of emission from below. 



The fine-grained outer layer of the Eskdale dyke shows 

 under the microscope no glass. It has been entirely devitri- 

 fied. Its most abundant mineral is the triclinic felspar 

 (probably labradorite) which occurs in thin prisms, ranging 

 fi'om ^^0 to less than yoVo of an inch in diameter. In the 

 specimens I have examined no large porphyritic prisms occur 

 among these minute forms. The augite may be observed in 

 its two common forms, minutely and irregularly granulat'ed or 

 drop-like, and also in well-defined crystals not unfrequently 

 twinned. Among the granules sometimes crystalline faces 

 and angles may be observed, and they always act on polarised 

 light, so that they are not mere glass. The augite prisms, 

 with well characterised forms, have sometimes a diameter of 

 tV of an inch, and thus attain a considerably larger size than the 

 felspar — a relation the reverse of that which I have usually 

 found to obtain among the dykes and intrusive sheets in 

 Scotland. The magnetite occurs in exceedingly minute octa- 

 hedra, which may average perhaps ^oVo of an inch in diameter. 

 They are found both singly and ranged in small rod-like 

 aggregations. I observed no olivine. On rotating a slice of 

 the rock between crossed Nicol prisms, no portion remains 

 persistently dark, but between the recognisable crystals inde- 

 finite fibres and granules make their appearance, which no 

 doubt represent the devitrification of an original glassy base. 

 A slight alteration from weathering, which the rock has 

 undergone, seems to have affected the indefinite ground-mass 

 more than the crystalline constituents. 



The central or more thoroughly crystalline portion of the 

 dolerite presents, under a magnifying power of forty diameters, 

 the general character represented in fig. 2 of PL VI. It is a 

 dolerite of the normal type among the Tertiary and Carboni- 

 ferous volcanic rocks of Scotland. At Shaw Burn this charac- 

 ter is well exhibited. The large, clear, and striated prisms 

 of labradorite are there the most conspicuous constituents of 

 the rock, when examined under the microscope. The augite 

 occurs in small prisms, which from their imperfection often 



