98 A NEW BRITISH ROCK. BY T. BARRON 



resemblance of these patches to those in the rock previously 

 described, there seems to be little doubt that they are com- 

 posed of nepheline. 



Eastee 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 por- 

 phyritic sanidines set in a base of felspar microlites, among 

 which is scattered a good deal of limonite evidently pseudo- 

 morphous after riebeckite, as it retains the exact outlines of 

 the original ophitic patch. The porphyritic 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 

 diflPerent 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 rock ; 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 Rock. — Macroscopically, this rock shows two sets of 

 bai^ds. One hq,lf consists of alternating bands of brown and 



