ON THE OLDER HOCKS OF ANGLESEY. 411 



it with, the arrangement of spicules in some sponges, and call it spicidar 

 arrangement. In the same district we find a more complex form (207) 

 (see fig. 22), which may really be the same rock under diSerent and special 

 circumstances. It is seen running into the crevices and wrapping round 

 the surface of a purple calcareous rock, and is characterised in the hand 

 specimen by a number of small pea-like bodies which are brought out on 

 its surface by weathering. The base of this is also a dusty substance with 

 minute polarising specks, in which are scattered very fine felspathic 

 microliths, which are in general irregnlarly placed, but in some places 

 they aggregate into sheaths, and elsewhere, particularly near the contact 

 ■with the shale, become complete spherulites, though without a circular 

 outline. None of this structure, however, represents the pea- like bodies. 

 These in a thin section form rounded areas, with a closely-set narrow 

 border of minute epidote crystals, with larger crystals of the same, or of 

 some other unrecognised mineral, filling the centre without any arrange- 

 ment. They vary in size from '005 inch upwards. No particular infor- 

 mation is afforded by this rock as to the origin of such varioles, as they 

 ought probably to be called, from their resemblance to some of the struc- 

 tures seen in the Variolites of the Durance. We can compare them also 

 with the epidote nests seen in the diorites near Newborough and in the 

 felsite at Mynydd Mechell. There are also in the rock several areas now 

 composed of an aggregate of epidote crystals, and having the external 

 form of orthoclase crystals. These appear to be altered insets. There 

 are also a few small patches of augite. 



A third remarkable spheroidal rock occurs as an isolated boss to the 

 west of Amlwch (227). The original structure of this can be best made 

 out with the paraboloid. We thus see that it contained a number of 

 acicular microliths, floating irregularly in what was probably a glassy 

 base, but it is now filled with infinitesimal particles of a shiny crystalline 

 appearance. These are arranged in relation to the microliths, often form- 

 ing a border to them, and sometimes appearing to radiate from them. 

 The whole mass is also separated into larger areas of very obscure and 

 iiTegular outline, which polarise as a whole somewhat after the manner 

 of the macrofelsite of Mynydd Mechell. But over and above all this the 

 rock has a special feature, which is the most striking one on first exami- 

 nation, but which is evidently of secondary origin. This is the develop- 

 ment of a crowd of small prismatic crystals of epidote, which stand 

 isolated or congregate in groups and have no regularity of arrangement. 

 That they are of later origin than the microliths is proved by their often 

 wrapping round and partially enclosing them. There are also a number 

 of calcitic patches and lines of ferric dust. 



It would seem from the structure of these rocks that an irregular 

 arrangement of long narrow crystals is the most favourable for the 

 development of large spheroids by weathering — the structure and the 

 form being alike peculiar, and in every case combined. It may be called 

 to mind that such is the structure of basalt, which is also noted for its 

 spheroidal weathering. The spherical surfaces are, of course, anterior to 

 the weathering, which only brings them out, and it is not suggested that 

 the spicular structure is the cause of these spherical shells, but that 

 the two are correlated as the double results of a single cause. If from 

 the circumstances of cooling, or the materials of the rock, a spicular 

 structure is brought about, it is probable that a spheroidal form will 

 follow. 



