A NEW BRITISH ROCfe. BY T. feARROlt" 9§ 



greyish -pink gV of an inch broad, the other of bands ^ of an 

 inch in thickness. Porphyritic sanidines, somewhat altered, 

 occur here and there, lying across and breaking the bands. 



Microscopically, the rock consists of a finely felted mass of 

 sanidine microlites with their long axes lying parallel to the 

 banding. Porphyritic crystals are rare, and when present 

 occur in granular aggregates. They are all more or less 

 altered, and show the irregular cracks across the prism so 

 characteristic of sanidine in the more recent trachytes. No 

 ferro-magnesian mineral is present in this rock ; but there is 

 evidence of its presence at an earlier period of its history, in 

 the ferruginous alteration-products which have been shown 

 to be formed from riebeckite in the other rocks. These 

 alteration-products have segregated along the flow-lines, thus 

 producing the beautiful banded appearance. 



The other rocks (unhanded) are very similar in microscopic 

 structure to that described above ; there is therefore no 

 necessity for a detailed description. 



Junction between Sandstone and Igneous Roch, on the South Side 

 of the Hill. — Macroscopically, the rock resembles the clay stone 

 porphyrites in appearance. It shows signs of minute spherulitic 

 structure near the juncture 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 been 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 



