IGNEOUS GEOLOGY OF THE BATHGATE AND LINLITHGOW HILLS. 135 



are lath-shaped, clear, and undecomposed, with few inclusions other than portions of the 

 glassy base, from which the terminations of the crystals and the basal planes are 

 frequently imperfectly marked off. Twinning both on the albite and the carlsbad laws 

 is very general, and from the extinction-angles there appears to be present a mixture 

 of plagioclases of the labradorite-andesine series. In the doleritic types a simply or 

 multiply twinned felspar in irregular plates seems to act to some extent as matrix. It 

 has invariably a lower refractive index than the lath-shaped felspars which it encloses, 

 and probably belongs to oligoclase. The augite occurs either in tiny idiomorphic 

 crystals or in small rounded granules, usually aggregated into little heaps. More 

 rarely in the dolerites it builds small ophitic plates between the felspars. The 

 magnetite occurs in idiomorphic cubes or open networks or feathery skeletal crystals. 



The glassy base is typically brown and microlitic, and when abundant imparts a 

 dark lustrous sheen to the hand specimen. In some cases it seems to be undergoing a 

 local clarification, with occasional development of anomalous refractive effects. The 

 discrimination of such material from analcite is frequently difficult, but in this and 

 similar cases it is convenient to adopt Evans' criterion, that in the absence of any trace 

 of crystalline structure the material should be considered hyaline.* In these rocks the 

 doubtful material, so far as observed, never shows a crystalline structure, while the true 

 analcite is so obviously secondary that it is very probable that no primary analcite ever 

 existed in them. The production of secondary analcite, which is sometimes very 

 abundant in the coarser-grained types, begins with the replacement of the felspathic 

 matrix, or of the intersertal glassy material by a yellowish granular isotropic substance, 

 which only later assumes the properties of analcite. The change can frequently be 

 observed advancing from its point of origin into the surrounding rock, the outlines of 

 the original lath-shaped felspars being occasionally recognisable within the brown iso- 

 tropic pseudomorph. (See PI. I. fig. 1.) It is highly probable that pneumatolytic action 

 as well as atmospheric weathering has been concerned in the production of the change. 



Structure, Classification, and Chemical Composition. 



The variations in structure are largely dependent upon the relative abundance of 

 felspar, augite, and glassy base in the groundmass, and as this affords also a basis for 

 classification, it is unnecessary to describe the structural modifications apart from the 

 general composition of the various members of the series. Five well-marked and 

 recurrent types are readily distinguished, each connected with the others by impercept- 

 ible gradations, the members of any one group varying slightly in texture amongst 

 themselves. 



1. Coarse-grained doleritic rocks, usually amygdaloidal and much decomposed, non- 



: porphyritic, or porphyritic with olivine alone, more rarely with augite. The felspar of 



the groundmass is more abundant than the augite which occurs in granules, granulitic 



* Evans, Quart. Joum. Geol. Soc, vol. 57, 1901, p. 49 ; see also Flett, Trans. Roy. Soc. Edin., vol. xxxix , 1900, 

 p. 865. 



