Alfred Sarker — On some Anglesey DijJies. 551 



The slide appears at the first glance totally unlike all the others, 

 showing only numerous flakes of mica in a confused mass of 

 decomposition products, without any trace of augite, olivine or 

 hornblende. The mica is less strongly coloured and less pro- 

 nouncedly dichroic than is usual in biotite, except in the bleached 

 mineral often met with in the lamprophyres and some peridotites. 

 It gives, however, a moderately deep or rather light brown for 

 vibrations parallel to the cleavage-traces, and a paler brown in the 

 direction at right angles to this. The extinctions are sensibly 

 parallel and perpendicular to the traces, and a cleavage-flake is dark 

 between crossed Nicols. The plates are split into thin lamellee 

 separated by lenticular grains and layers of a colourless, doubly 

 refracting mineral. The interposition of calcite lenses between the 

 folise of biotite has been noticed by Zirkel, Hussak, and Williams, 

 but here the mineral is not calcite. Each lenticular grain consists 

 of portions differently oriented, and there is sometimes a radial 

 fibrous appearance suggestive of a zeolitic mineral. The mica 

 includes also a large quantity of granular magnetite. A slide [711] 

 from the Llanbabo mass contains precisely similar flakes of mica, 

 though in much less quantity, and the mineral appears to be closely 

 connected with the brown hornblende, as if resulting from its 

 alteration. This slide probably shows the beginning of a process of 

 which the Llandyfrydog specimen represents the final stage. The 

 secondary production of biotite in place of hornblende is seen also in 

 the Pen-y-rhiwiau and Penarfynydd rocks mentioned below, and in 

 some other "Welsh rocks, such as the syenite of Llanfaglen near 

 Carnarvon. 



The structural variations of each dyke have not been studied in 

 detail. As Henslow and Professor Hughes have pointed out, the 

 megascopic characters often change largely in a very short distance, 

 though the microscope shows that these variations do not always 

 import any considerable difference in constitution. The more 

 felspathic type of rock sometimes occurs as segregation-veins in the 

 darker and more hornblendic. The dykes grow rather finer-grained 

 at the immediate margin, and these portions are more susceptible to 

 destructive weathering action than the coarser rock. A slide (A 134:) 

 from Llys Einion, close to the junction with the shales, shows 

 advanced decomposition ; but besides the calcite, ' viridite,' etc., there 

 remains an abundance of felspar in good crystals, and partly bleached 

 brown hornblende in crystals and ophitic plates, the latter bordered 

 by colourless fringes of the usual secondaiy growth. 



The coarse black type of rock, with its cement of secondary 

 hornblende, is very durable, and, as has been remarked, the erratics 

 are mainly of this variety. The dykes themselves form prominent 

 features in the country, which doubtless led Henslow to map the 

 whole district north of Llanercliyraedd as consisting of these rocks. 

 The smaller dolerite dykes of southern Anglesey, on the other hand, 

 make but little show at the surface, and are difficult to find ; a fact 

 indicated on the Survey Map, where the greater part of the dykes 

 marked are on the sea-shore. 



