Alfred Marker — Some Anglesey Dykes. 413 



law, is observed. There is also, as in the Caclnant dyke, a later 

 generation of felspars, shapeless, but broader than the others. They 

 are sinaple or once twinned, and between crossed Nicols show strong 

 zonary shading : as usual they are clearer than the earlier felspars. 

 The augite is in light-brown plates, moulding the dominant felspars : 

 it shows branching layers of interpositions. The chief secondary 

 products in this slide are a yellowish-green sti'uctureless substance 

 and a ferruginous staining ; looth seem related to the augite. 



[486.] Dolerite ; the more compact portion of the dyke ; a fine- 

 grained ophitic rock with abundant magnetite, closely resembling 

 [546] from the Cadnant dyke. The felspar is in elongated prisms, 

 with mostly repeated albite-twinning and fairly wide extinction- 

 angles. It is clear except for some granular calcite and fine viridito 

 strings, following for the most part, the basal cleavage-cracks. 

 Magnetite is plentiful in crystals, skeletons, and rods ; and is slightly 

 posterior to the felspar. The apparent rods, which are probably 

 sections of plates, have a parallel disposition, and are arranged trans- 

 versely to felspar crystals which they surround. A similar relation 

 of magnetite to olivine is described by Keusch from the basalts of 

 Jan Mayen.'' The present rock contains also minute rings (spherical 

 shells) of magnetite dust surrounding nuclei of the same. The augite, 

 always allotriomorphic, has the same characters as in the preceding 

 slide. Chloritic decomposition-products and clear secondary quartz 

 occur. 



The metaraorphic effects of the Plas Newydd dyke upon the 

 adjacent Carboniferous strata, as described by Henslow, possess 

 considerable interest. On the south-west side a bed of calcareous 

 shale, abutting upon the dyke, is converted into a kind of lydianite, 

 containing calcite and clusters of garnet and analcime crystals. 

 Henslow's specimens [511 to 523] show the development of these 

 last-named minerals in every stage from mere whitish concretionary 

 spots in the hardened shale to perfectly formed crystals. These are 

 closely clustered together along particular bedding-planes, and some- 

 times the valve of a Productus, converted into crystalline calcite, is 

 seen to be studded over and penetrated by globules or crystals of 

 analcime. 



The garnet occurs in crj'stals up to 0'7 inch in diameter, showing 

 the faces of the rhombic dodecahedron (110), sometimes truncated 

 by narrow planes belonging to the form (211) : they have often 

 a marked concentric zonary structure. They vary from yellowish- 

 green to olive-brown, with a resinous lustre. " Their specific gravity 

 is 3'353," and their hardness under 7. These characters indicate 

 a variety approaching grossularia ; Lyell ~ gives the percentage of 

 lime as 20. A slice (cut from a sj)ecimen in the Sedgwick collection) 

 shows a number of crystals in various stages of development, closely 

 packed together, and the interstices filled with crystalline calcite. 

 The crystals are not all isotropic : they contain a large amount of 

 foreign material. The readiness witli which garnets of considerable 



1 Cf. also Prof. Judd's basalt from Mull, Q.J.G.S. vol. xlii. pi. vi. fig. 7, 1886. 



2 Student's Elements of Geolojy, crd ed. p. 513. 



