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in a partial bleaching of the rocks, in their induration into a flinty 

 substance, and in the development of a microcrystalline structure in 

 them. This alteration has been effected partly along the planes of 

 bedding, and partly across them. The feeblest degree of chauge is 

 marked by a slight induration of the shale or grit, the clastic nature 

 of the rock being still obvious. Prom this condition successive 

 stages may be traced until the rock appears in milk-white, flint-like 

 masses, homogeneous, translucent, and breaking with a splintery to 

 conchoidal fracture. 



These changes are most conspicuously seen in the uppermost zone of 

 the volcanic group, but are not confined to it. They appear wherever 

 the porphyries have invaded the rocks. The best locality for their 

 study is the coast-section at Nun's Chapel, where they attain a re- 

 markable development in the zone of fine tuffs and shales below the 

 quartz conglomerate. They are shown also among the corresponding 

 strata at Ogfeydd-duon, on Eamsey sound. At first the latter 

 locality seemed to be an exception to the rule that this kind of 

 metamorphism is connected with the protrusion of the quartz por- 

 phyries ; but, searching the ground in the neighbourhood, I after- 

 wards found the prominent and massive porphyry crags of Treginnis. 

 On a lower horizon the alteration has been well developed in the 

 agglomerate of Clegyr, where also there are intrusive dykes of sphe- 

 rulitic porphyry. And, on a still lower platform, similar induration 

 accompanies the quartziferous porphyry, of the Board Schools. 

 From the published descriptions, the association of highly siliceous 

 bands (porcellanite, halleflinta, kieselschiefer, adinole, or whatever 

 they may be called) with masses of felsite and quartz porphyry 

 would appear to be of common occurrence in Wales. 



The sections that exhibit most clearly the metamorphism asso- 

 ciated with the porphyries are those which have been cut by the sea 

 along the coast from Nun's Chapel eastward. In the first stages of 

 change the shales are indurated, begin to lose the distinctness of their 

 bedding, and break with a splintery fracture. Gradually they become 

 feebly translucent on the edge, like the porcellanite or kieselschiefer 

 of contact metamorphism. The granular texture passes into one like 

 that of hornstone, and the edges become more translucent, until, losing 

 by degrees all obvious trace of clastic structure, the rock presents a 

 translucency, fracture, and lustre like those of flint or chert. The 

 colours of these various conditions of the siliceous material range 

 through shades of dirty grey and bluish and greenish grey to milky 

 white. The alteration having been developed more particularly along 

 the bedding of the strata, the indurated layers appear mostly as 

 bands interstratified with the schists, shales, or tuffs. So evenly, 

 indeed, are these layers interposed that they may readily be regarded 

 as original deposits, formed contemporaneously with the strata 

 among which they lie. They vary from thin lamina to bands a foot 

 or more in thickness. Some of them arc regularly banded in alternate 

 layers of more granular and more flinty texture. It is deserving of 

 remark that, owing to the tilted position of the beds, the indurated 

 bands are usually highly inclined or vertical, presenting occasionally 



