Alfred Harker — Some Anglesey Dykes. 411 



Cordier pronounced some specimens from this part of the Straits 

 to be " dolerite. The pyroxene very evident, with fer titane." In 

 the slides examined, however, the iron-ores never show forms 

 characteristic of ilmenite, and no leucoxeno occurs. The compact 

 rocks are named " basaltic lava." 



Cadnant dyhe. — Between Garth Ferry and Menai Bridge we meet 

 with the first large dyke, which occurs on the right bank of the 

 little stream at Cadnant. This dyke is not marked on the Maps of 

 the Geological Survey, but Henslow traced its course among the 

 schists for some distance inland. He also found it on the Carnarvon- 

 shire shore, where it cuts Carboniferous strata. In the latter place 

 the rock is less compact in texture than on the Anglesey side, and 

 Henslow states that "none of the d^'kes which intersect the lime- 

 stone and shale attain to so great a degree of compactness as the 

 generality of those which are found among the schist." Three 

 dykes having a similar strike to that at Cadnant are marked on the 

 Survey Map south of Bangor Station, and a slide from one of these, 

 at Glan Adda, has been examined. Cordier identified the Cadnant 

 rock as " a true dolerite, having the ingredients, pyroxene, fer 

 titane, and felspar well characterized." 



[545.] Dolerite ; a typical specimen from Cadnant. — To the eye 

 the rock shows a moderately coarse-grained aggregate, in which the 

 augites are conspicuous, moulding elongated crystals of felspar. 

 Under a low objective the structure is seen to be holocrystalline and 

 ophitic. The felspars occur in mostly elongated sections, showing 

 a rather fine albite-twinning, combined with a concentric zonary 

 banding, and sometimes twinning on the pericline law. From the 

 extinction-angles they should be andesine or oligoclase. This rock 

 exemplifies a character found in most other similar dykes in North 

 Wales ; the felspars appear to be of two generations, of which one 

 is older, and the other newer than the bulk of the magnetite. The 

 later felspars are distinguished by their imperfectly defined outline 

 and more equal dimensions, their more marked zonary banding, and 

 their greater clearness : their twin-lamellce are usually very narrow 

 and rather wide apart. A few prisms of apatite are seen. The 

 magnetite in this slide is plentiful, and builds crystals of intricate 

 branching shapes. The augite has the pale-brown tint common to 

 that mineral in almost all North Welsh rocks. It moulds the other 

 constituents, and never shows crystal boundaries. The prismatic 

 cleavages are strongly marked, and there are ill-defined interpositions 

 arranged in branching lines or planes. The chief secondary product 

 is a yellowish-green, doubly refracting mineral, which pseudomoi'phs 

 augite : secondary granular magnetite accompanies it in places. The 

 dominant felspar is often almost opaque owing to alteration-products. 



Porphyritic dolerite : Glan Adda, south of Bangor Station. — This 

 dyke occurs on the same line as that at Cadnant, and may be briefly 

 referred to for comparison. Here the earlier felspars have lath- 

 shaped sections : the later ones are more equi-dimensional, and show 

 the zones of different chemical composition very distinctly. The 

 augite moulds the older felspars, but is of earlier formation than the 



