132 PKOCKEDINGS OP THE GEOLOGICAL SOCIETY. 



from a rock largely composed of altered olivine into one consisting 

 mainly of felspar with but little olivine. And even among the 

 Carboniferous sills, Dr. Stecher has ascertained that the marginal 

 portions which cooled first and rapidly, and may be taken, there- 

 fore, to indicate the mineral composition of the rock at the time of 

 extrusion, are often rich in olivine, while that mineral may be 

 hardly or not at all discernible in the main body of the rock.^ 



Some of the rocks of this group are holocrystalline, like the Cor- 

 storphine Hill sheet, but more frequently they show more or less of 

 a devitrified groundmass. They exhibit the much more largely 

 crystalline structure of intrusive sheets as compared with superficial 

 lavas. 



(c) In the field, Dolekite and Basalt without Olivine (Olivine- 

 EREE Diabase) are not to be distinguished from the rocks just 

 enumerated. They occur under precisely similar conditions, and 

 show a corresponding range of variation from holocrystalline rocks 

 to others with a marked proportion of devitrified groundmass. In 

 the Eatho rock, as Mr. Teall has pointed out, micropegmatite 

 plays the part of interstitial matter.^ Other varieties have been 

 discriminated by Dr. Hatch, having their mesh of lath-shaped felspar 

 filled in with granular augite, magnetite and base, as in tholeiite. 

 He further distinguishes another type presenting rather large lath- 

 shaped felspars, here and there penetrating ophitic orthoclase, and 

 with granular augite (Burntisland sills). In one type the ophitic 

 structure is developed ; in another the coarse-grained dolerite shows 

 large felspars in a base which was once glassy (Muckraw, Linlith- 

 gowshire). Many large sills of coarse dolerite exhibit a tendency 

 towards an ophitic structure, the felspars penetrating the augite, 

 but contain besides more or less quartz, sometimes also orthoclase, 

 and even large patches of a glassy base (Bowden Hill and Queens- 

 ferry, Linlithgowshire). As I pointed out many years ago, some 

 of the sills in West Lothian contain bitumen and give off a bituminous 

 odour when freshly broken. They have been injected into bitu- 

 minous shales or coal-seams.^ 



(d) Pohphieite appears to be of as rare occurrence among the 

 intrusive as among the interstratified parts of the puy-series. Some 

 of the necks, and possibly sills, in the Limerick basin consist of it. 



1 Stecher, Tschermak's Mineralog. Mittheil. vol. ix. (1887) p. 193. Proc. 

 Eoy. Soc. Edin. ^^ol. xv. (1888) p. 162. 

 ^ * British Petrography,' p. 190. 

 3 Geol. Survey Memoir on Geology of Edinburgh (Sheet 32, Scotland), p. 46. 



