252 Proceedings of the Royal Physical Society. 



on being submitted to microscopic examination prove to be 

 aggregations chiefly of crystalline grains and imperfect prisms 

 of augite with very small triclinic felspars, and also scattered 

 magnetite. They have no definite external contour. The 

 crystals project from them into the surrounding glass, which 

 can also be traced among interstices even in the midst of the 

 kernels. These patches therefore are original aggregations 

 of the components of the rock, and not extraneous included 

 fragments. 



The Magnetite presents no features requiring special remark. 

 It is well crystallised in the forms usual in basalt, but on 

 the whole is less abundant than in many of the basalts of 

 central Scotland. Its smaller octohedra are crowded round 

 the augite prisms, forming here and there a kind of black 

 border. Its dust-like grains appear in the most globulitic 

 parts of the glass, and it is probably they that encrust 

 some of the minuter forms of microliths, which consequently 

 appear as opaque rods or needles. 



I have not succeeded in detecting olivine in any of 

 the slices yet prepared. There occur indeed here and there 

 rounded kernel -like aggregations of dull green serpentinous 

 matter whicli may represent this mineral. But they are 

 comparatively infrequent, so that even if olivine be present 

 it must take but an insignificant part in the constitution of 

 the rock. 



(3.) Chemical Composition of the Eshdale Bock. 



To complete this account of the so-called " pitchstone " of 

 Eskdale, there remains for consideration the question of its 

 chemical composition. It is evident that while the rock is 

 not to be associated with the acid pitchstones, neither on 

 the other hand are its affinities to be sought among the more 

 basic basalts. It is well known that among the vitreous 

 forms of the basalt-rocks varieties occur with a much more 

 acid character than normal crystalline basalt. If we take 

 the analyses recently collected by Eoth * of the crystalline 

 felspar-basalts, the proportion of silicic acid may be set down 



* Beitrage zur Petrographie der Plutonischen Gesteine Abhandl. k. Akad. 

 Wissensch. Berlin, 1869, p. 195. 



