IGNEOUS GEOLOGY OF THE BATHGATE AND LINLITHGOW HILLS. 145 



"quartz vermicule " of Levy and Lacroix.* Where the growth has assumed a 

 crystalline form, as occasionally happens within the intersertal spaces, it is invariably 

 that of felspar, frequently simply twinned, and occasionally with an hour-glass structure 

 similar to that found in augite."'' Sometimes also the micropegmatite may be observed 

 eating into the earlier columnar felspars, partially or wholly replacing one half of a 

 narrow carlsbad twin, or attacking the earlier crystal in a number of isolated spots. 

 The felspar of the micropegmatite has a refractive index commonly lower than that of 

 the quartz, and is apparently always an alkali felspar. It is rarely found absolutely 

 fresh. Usually it is much kaolinised, and it is, in fact, to the turbid condition of the 

 mesostasis, sometimes stained with iron, that the grey or reddish colour of scattered 

 portions of these rocks is largely due. 



The Structure of the Diabases. 



Two factors have apparently influenced the structure of the diabases, namely, the 

 period of maximum crystallisation of the felspars and the amount of mesostasis at 

 different points within the mass. Where the felspar has, on the whole, crystallised 

 before the augite, while mesostasis is entirely absent or in exceedingly small amount, a 

 true noncrystalline ophitic structure has been produced. This type, however, is com- 

 paratively rare, and mostly confined to the marginal portions of the larger sills. In the 

 dykes by far the commonest type is that which may be described as intersertal-sub- 

 ophitic, for the production of which a fair amount of mesostasis is required, and a slight 

 extension of the period of the felspar both before and after that of the augite. Here 

 the augite assumes the peculiar habit already described on p. 140. Where the bulk of 

 the felspar has crystallised after the augite, and a large amount of mesostasis occurs, as 

 in the central portions of the sills, both minerals develop long columnar forms. The 

 augites may enclose marginally small early felspars, but, as a rule, they are moulded and 

 more or less corroded by the later felspars. The large amount of mesostasis has here 

 apparently given scope to the felspar to develop idiomorpbic outlines wherever they did 

 not come into close contact with the augite. A rude parallel arrangement of the 

 columnar felspars and augites is sometimes to be observed in the interiors of the sills, 

 and this, with the bending and breaking of the augites already referred to, is doubtless 

 due to the effect of the pressure of the overlying rocks upon the partially solidified 

 igneous material. 



The Diabase Aphanites and Diabase Porphyrites.| 



Towards the margins of the dykes and sills the coarse-grained diabases pass gradually 

 into compact, non-porphyritic aphanites, frequently spotted with dark green ocelli. At 

 the contact itself the aphanites pass into exceedingly fine-grained and porphyritic 

 basaltic varieties. Glassy modifications appear to be absent. 



* Lacroix, Mineralogie de la France, vol. ii. p. 36. Of. also " Myrmekitic Structure," Weinschenk, Die gesteins- 

 bildenden Miner alien, p. 75. 



t Flett, Trans. Geol. Soc. Edin., vol. viii. p. 485. \ Zirkel, Lehrbuch der Petrographie, vol. ii. p. 699. 



