189 



twinned on the albite plan, and the high extinction angles show that 

 they belong to a species allied to anorthite. The augite is distinctly 

 pleochroic in reddish brown and yellow tints. The olivine is markedly 

 yellow in some sections (hyalosiderite), and passes into a rich deep 

 green serpentine. This is an important and constant feature in the 

 basic eruptive rocks of Britain, and is well illustrated also in the 

 dolerites of the Midland Counties of England. Colourless olivines 

 weather into pale green or yellow serpentinous products ; yellow olivines, 

 that is olivines which appear yellow in thin section (hyalosiderites), 

 weather into deep green or orange yellow alteration products. Another 

 feature in the fresh olivines of this rock is the occurrence of cracks 

 containing segregations of magnetite. The ground-mass is essentially 

 similar in composition to that of the Lion's Haunch rock, but differs in 

 the fact that the small augites constantly occur as extremely minute 

 and well formed crystals which lie embedded in a matrix of felspar. 

 These augite microlites are prismatic in form and are often many times 

 longer than broad. 



The rock of Sampson's Ribs is similar to that of the Long Row in its 

 petrographical characters. It is a porphyritic basalt. 



We have now to notice the rocks which occur as intrusive sheets 

 in the neighbourhood of Arthur's Seat. 



Salisbury Craig is formed of a medium grained granular rock described 

 by Mr. ALLPORT as containing " visible crystals of felspar and augite 

 with numerous grains of calcite, analcime, and two minerals which 



appear to be prehnite and pectolite Thin slices exhibit under the 



microscope considerable variations in texture ; in all, however, there are 

 crystals of plagioclase, orthoclase, [?] augite, magnetite and apatite with 

 more or less of glassy base. The augite varies in shade from light brown 

 to purplish brown, the latter being slightly dichroic ; in the more highly 

 altered specimens this mineral is converted wholly or in part into pseudo- 

 morphs of a greenish brown granular substance. Those portions of the 

 rock which contain calcite and zeolites may have been slightly vesicular 

 originally ; but the mode of occurrence of these substances seems 

 rather to indicate that they have replaced a glassy or felsitic matrix 

 [pseudo-amygdules]. In some specimens comparatively large spaces 

 are filled with calcite and analcime ; the latter may easily be mistaken 

 for the glassy base, as it belongs to the tesseral system, and is there- 

 fore devoid of double refraction/ 1 ) It occurs, however, in distinct 

 crystalline forms of eight sides, which are no doubt sections of trapezi- 

 hedrons the most common form of the mineral. Chlorite in vermicu- 

 lar crystals and also grouped in rosettes, occurs in the calcite and analcime, 

 and is also disseminated throughout the mass. Some specimens of this rock 

 are far more highly altered than others." 



The rock of the irregular ridge known as the Dasses is rendered 

 conspicuously porphyritic by the occurrence of large felspar crystals in a 



(1) This mineral occasionally shows anomalous double refraction like that of many other 

 crystals which are isotropic at the of time their formation. 



