T. Barron — A British Rock containing Nephelinc, etc. 377 



Junction between Sandstone and Igneous Roclc, on the South Side 

 of the Hill. — Macroscopically, the rock resembles the claystone 

 porphyrites in appearance. It shows signs of minute spherulitic 

 structure near the junction with the sandstone. The line of junction 

 is not straight and sharp, but the igneous rock has sent small 

 offshoots into the sandstone. It is thus highly probable that the 

 sandstone was in a loose unconsolidated state when the lava was 

 poured over it. 



Microscopic examination of a section shows the trachyte to be 

 a good deal altered. It has evidently been originally a glassy 

 rock containing a few porphyritic crystals of sanidine which are 

 now replaced by chalcedony. The glass, which is now largely 

 devitrified, is crowded with altered spherulites. The devitrified 

 base has the appearance of the highly siliceous ground-mass of the 

 felsophyres of the acid rocks. It is thus highly probable that 

 a good deal of quartz has beeii fused and incorporated in the lava 

 along the line of contact. 



The sandstone also shows signs of fusion along the line of contact 

 with the trachyte. The sand-grains are cemented together by a 

 finely crystalline aggregate of quartz. Along the junction-line, 

 the grains show much corroded outlines, and many of them show 

 a ring of fused material having very indefinite boundaries. Where 

 the trachyte has sent veins into the sandstone, lines of fusion are 

 seen which much resemble the fluxion-structure of a rhyolite. 

 Kound these veins the quartz has been fused ; it is now in the 

 form of chalcedony. In the sandstone itself, mica is seen to be 

 developing ; and there is a good deal of felspar, mostly unaltered 

 plagioclase showing lamellar twinning. 



Another specimen of sandstone, taken one foot below the junction- 

 line, had lost all traces of bedding, aud broke with an irregular 

 fracture. Scales of a silvery mica were seen lying in all directions 

 and at all angles to the bedding-planes, thus proving that they have 

 been developed subsequent to the deposition of the sandstone. 



Under the microscope, the mica proved to be muscovite. Fresh 

 plagioclase has also been developed ; and the cementing material 

 between the sand-grains has been made to assume a finely granular 

 form. 



The presence of fresh plagioclase in a sandstone is not an every- 

 day occurrence, and it is necessary to find an explanation to account 

 for it. Sandstone, being a porous rock, is not likely to contain the 

 alkalies necessary for the formation of plagioclase felspar, because 

 of their solubility in water. They must, therefore, have been 

 derived from another source, that being the overlying igneous rock. 

 It was noticed that no ferro-magnesian mineral was observed in the 

 overlying rock ; but there was a good deal of iron oxide scattered 

 through it, which was shown to be derived from the alteration of 

 riebeckite in the Eildon rock. This mineral, being a soda-amphibole, 

 might, by its decomposition, supply the alkali necessary for building 

 up a felspar such as albite ; and I am inclined to regard this as 

 the true explanation of the presence of fresh plagioclase in the 

 sandstone. 



