■60 Jutson and (.-li airman : 



lender a high power of the microscope, thin sections (Plate VI., 

 fig. 4) are seen to consist of a felsitic ground mass with altered 

 felspars giving a shadowy extinction, small blebs of quartz, numerous 

 tufts of sericitic mica, and occasional pale green tourmalines. On 

 the field evidence the rock would be classified as an acid dyke. 

 Under the microscope, however, on account of the decomposition 

 that has taken place, it might possibly be regarded either as a 

 fine-grained dyke-stone or as a volcanic ash : but from a considera- 

 tion of both the field and microscopic evidence, we are of opinion 

 that it is a fine-grained decomposed felsite. 



The selvage rock from which the slide (Plate VI., fig. 3) has been 

 taken is a brick-red jaspery breccia. Some of the included frag- 

 ments measure as much as 11 mm. in diameter. These seem to have 

 been originally sandstones and secondary quartz vein-stone. 

 ■Certain of the quartz fragments are distinctly rounded (due prob- 

 ably to crushing), and some show characteristic polysynthetic struc- 

 ture under crossed nicols. The brecciated rock-fragments are 

 -cemented by a ground mass of fine felsitic material, which has a 

 purplish or reddish tint owing to the quantity of haematite included 

 in it in segregation patches, and bordering the larger fragments 

 of polysynthetic quartz. Sericite is abundant in minute flakes in 

 the matrix of the rock, and traces of a large simply twinned ortho- 

 ■clase on the (?) Baveno type also occur. Many of the secondary 

 quartz fragments show interesting stress shadows as well as micro- 

 gneissic structuie. Macroscopically this rock resembles a volcanic 

 "breccia, with included fragments of sedimentary Silurian torn from 

 the rocks adjacent to the fisstire. This term, however, generally 

 implies a pyro-clastic rock formed by an explosion from a volcanic 

 vent, which does not appear to be the case with the rock in question. 

 It is better described simply as a felsitic breccia. It may have been 

 formed either by the dyke on its intrusion tearing off fragments of 

 the adjacent rocks, which became mixed with the dyke-stone at or 

 near the margin of the dyke, or by earth movements causing the 

 dyke to be brecciated subsequent to its consolidation. As the dyke- 

 stone appears to form the matrix of the brecciated selvage, and 

 the rocks have no distinct boundary line, but are irregularly fused 

 together, we incline to the former explanation. 



A certain amount of mineralisation of the rocks, paiticularly this 

 selvage rock, has taken place, as shown by the thin veins and 

 patches of pyrites and other minerals in hand specimens. 



The enamelled rock is of an ochreous yellow to a brown colour. 

 It resembles a fine-grained laminated ash breaking up into several 



