Alfred Marker — On some Anglesey/ Dykes. 549 



The cross-section always shows the prism form (110) well developed, 

 often truncated by the clinopinacoid (010), and sometimes the 

 orthopinacoid (100). Twinning on the usual law is common. The 

 prismatic cleavages are well-developed, and rarely the clinopinacoidal 

 cleavage is nearly as well marked. The colour is usually brown, 

 with strong pleochroism, vibrations parallel to the axes of elasticity 

 giving : 7, deep chestnut brown ; ft, a less deep brown ; a, a pale 

 brown. The absorption is then r^ ■:> ft > > a; more rarely it is 

 r^/ = ft > > a. Sometimes, however, this original hornblende is 

 brownish-green, the vibrations parallel to the 7-axis giving a pale 

 grass-green, and perpendicular to it a very pale brown. The larger 

 crystals occasionally present a mode of alteration different from the 

 ordinary chloriticand serpentiuous changes, and consisting sometimes 

 of a brown coloration, sometimes a decoloration and decomposition 

 into a granular substance. These alterations affect the hornblende 

 along certain planes, which are not parallel to the prismatic cleavage, 

 but to the clinopinacoid, and are perhaps to be regarded as " solution- 

 planes" (Losungsfldchen). 



Another kind of hornblende, probably also original in the usual 

 sense, is often present in the dykes to the north of Llanerchymedd. 

 A cross-section of a brown hornblende crystal is seen, showing the 

 prism and clinopinacoid faces, but upon the latter a later accretion of 

 green hornblende-substance, in crj^stalline continuity with the brown, 

 has reduced or entirely built over the clinopinacoids, extending the 

 prism-planes at their expense. From its relation to surrounding 

 minerals and from its always presenting definite crystal boundaries, 

 this later growth of green hornblende seems to have originated 

 before the final consolidation of the rock. 



Next comes the hornblende resulting from the alteration of 

 original augite. The " paramorphic " origin of hornblende from 

 pyroxenic minerals has received so much notice in recent years, 

 that there is no need to enlarge upon it here, further than to remark 

 that the term " paramorphism " is strictly applicable only when the 

 two minerals yield identical analyses, which is probably an unusual 

 case. In tlie present instance it is most likely that some chemical 

 as well as physical change is involved. The hornblende thus produced 

 is brown, compact, and well-cleaved : in fact a study of the Anglesey 

 dykes, as well as rocks from several localities in Carnarvonshire, 

 bears out the conclusion of Lessen and others, that neither colour 

 nor structui-e affords any reliable criterion for discriminating between 

 original hornblende and that resulting from the amphibolisation of 

 augite. It is possible that some of the hornblende may be derived 

 from a rhombic pyroxene, and indeed the slides from the rocks east 

 of Llanbabo have a mineral resembling altered enstatite. Professor 

 Bonney has noticed the same in the Llys Einion dyke and boulders. 



Some slides (e.g. from the Hafod-in-in dyke) contain a consider- 

 able quantity of a rather pale, dull, brownish-green or gi-eenish- 

 brown hornblende without crystal boundaries. Compared with the 

 foregoing varieties, it is seen to be less clear and less strongly 

 coloured, to give lower polarisation-tints, and to have its cleavage- 



