ON THE OLDER BOCKS OF ANGLESEY. 391 



no case in tliis rock is there the slightest evidence of anything between 

 one element and another, nor of the presence of any other mineral than 

 quartz. When seen on the large scale there are appearances of a previous 

 tine banding in some parts of the rock, and of these being subsequently 

 broken up, and recemented by fresh quartz. One such band is recognis- 

 able in the slide. It is composed of rather clearer elements, followed by 

 a crack, on the other side of which the elements are smaller. This crack 

 is thrown into such deep sigmoid folds that, if ever it was straight, the 

 rock must have been greatly compressed ; yet the only sign of pressure is 

 a slight amount of spectral polarisation. The rock is also traversed by 

 veins, in which the elements are clear, but are nevertheless in optical 

 continuity with the previously-formed elements on either side. These 

 veins were subsequent to the folding of the crack, as the crests of the 

 sigmoids are cut off by them. The rock has, therefore, had a long 

 history, and the earliest form traceable is the polygonal structure. If 

 now we compare this with the structure of a jasper, such as is associated 

 ■with the limestone at Cerrig Ceinwen (95), we find in the latter also the 

 whole mass, with the exception of the fine ferric dust, to be siliceous and 

 mostly quartz, but the elements are small and irregular, though with 

 sutural boundaries. In this case we are sui-e that the rock has been pro- 

 duced directly from siliceous waters, and the chief difference between the 

 two is in the size of the elements. There is just the same difference 

 between the elements of a granite and a felsite of the same composition. 

 It is possible, therefore, that there may be the same relation between the 

 quartz rock and the jasper as between these two. In other words, the 

 quartz rock was more slowly formed, and at greater depths beneath the 

 surface. From these considerations, from the banding of portions of the 

 rock, from its purity, and from the mode of its occurrence it has been 

 suggested that such a knob represents the underground base of a hot 

 spring of the period. 



If this is the most satisfactory explanation of this particular rock it 

 must be allowed to have its weight in the interpretation of the others, 

 which resemble it in mode of occurrence, but which present greater diffi- 

 culties. The purest of these occur also in the Northern district, and have 

 been examined from Bull Bay (231), Port Unal (214), and Pen-bryn- 

 yr-Eglwys (36). In these the elements are not so uniformly large, 

 though the larger ones have equally sutural boundaries and polygonal 

 outlines ; but there is also a considerable quantity of finer elements irre- 

 gularly dispersed, and some of them have a minute sericitic boundary. 

 There are, moreover, in the two first some rare and minute zircons, and 

 in the last some certainly derived fragments which are not of quartz. 

 Still the rocks are essentially pure quartz, of which by far the greater 

 part is certainly authigenetic. Although it is difficult to account for so 

 great purity in a sedimentary rock, Hke an ordinary quartzite, it is not 

 difficult to account for the slight impurities that might be present in the 

 deposit from a siliceous spring. 



In the case of the other quartz knobs which have more numerous 

 derived fragments, these still have a special character, though, no doubt, 

 if it were not for the peculiar stratigraphy, and the existence of purer 

 quartz rocks having the same form, some of these might well pass for grits. 

 If we examine the quartz rock of Bethel, Bodorgan (96), we find that 

 the most conspicuous of the derivative elements are quite peculiar. They 

 are larger than all the rest, and their outlines are very rounded, and they 



