376 T. Barron — A British Rock containing Ncp/tcline, etc. 



Easter Eildon. 



The rocks composing this hill are typical trachytes. In hand- 

 specimen they show, on a fresh fracture, the cleavage- faces of a 

 sanidine felspar, in some cases slightly kaolinized ; they have the 

 general rough appearance and feel which characterize trachytes. 



Under the microscope the rock is seen to consist of porphyritic 

 sanidines set in a base of felspar microlites, among which is 

 scattered a good deal of limonite evidently pseudomorphous after 

 riebeckite, as it retains the exact outlines of the original ophitic 

 patch. The porphy ritic sanidines are resolved into granular aggregates 

 between crossed nicols. In some of these there has evidently been 

 a growth of the crystal after the consolidation of the rock, as there 

 is evidence in some cases of an attempt to assume a crystal form. 

 A notable feature of the felspars of this rock is the number of 

 sections which give a biaxial figure in convergent polarized light. 

 Some of the larger crystals have been dissolved away and their 

 places filled up by secondary quartz. Sections of the rocks from 

 other parts of the hill did not show any very different characters; 

 a special description is therefore unnecessary. 



Between these rocks and that from Ailsa Craig described by 

 Mr. Teall, there is a great similarity, Their ground-mass is almost 

 identical with that of the Ailsa Craig i - ock ; the riebeckite is in 

 ophitic patches, and shows the alteration into pseudomorphs of iron 

 oxide in both cases ; and, except for a larger quantity of quartz 

 in the latter rock, the resemblance is very striking. If similarity 

 of structure and composition be of any value in determining the 

 age of a rock, perhaps the fact of the rocks just described being 

 of Upper Old Red Sandstone age may help to suggest the age of 

 the Ailsa Craig rock. 



Black Hill. 



Banded Roclc. — Macroscopically, this rock shows two sets of bands. 

 One half consists of alternating bands of brown and 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 rooks. 

 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. 





