ON THE OLDER KOCKS OF ANGLESEY. 377 



most conspicTions feature of the rocks, and yet not every sign of the 

 original deposit has disappeared. The last things to disappear are 

 the large fragments, which are usually of felspar, and often lie in 

 a transverse direction to the general orientation. That these frag- 

 ments are not ' eyes,' in the sense of being the only parts of a felspathic 

 rock which has escaped destruction by pressure, may be gathered from 

 their general occurrence also in the rocks already described, especially in 

 the laminated rocks, and the probability of the origin in the two cases 

 being similar. It may be added that there is usually no sign of pres- 

 sure in connection with these fragments. They may rather be accounted 

 for by the difticulty of their absorption and recrystallisation, owing to 

 their size. The following may be cited as illustrations of these solitary 

 fragments. In the cleaved rocks of Roscolyn {67) there are many 

 large pieces of quartz and mica. In the Northern district the rock at 

 Llanflewin (210) has rather small fi-agments, including plagioclase. In 

 the Central district only one out of five of the highly altered rocks ex- 

 amined contained any original fragments — namely, that at Porth-gwyfen 

 (76), and these are of speckled felspar. In the Eastern district, also, only 

 a small proportion — five out of twenty-seven examined — show any frag- 

 ments ; and in only two are the fragments at all conspicuous — namely, 

 in peculiar rocks near Berw Tcha, south of Holland Arms (152). These 

 fragments are of quartz. 



From the above descriptions it will be seen that a large proportion of 

 the older rocks of Anglesey, obtained from all parts of the series, still 

 retain unobliterated their original structure, though it has been altered 

 by later actions of several kinds. These original structures vary, as do 

 the ordinary deposits of later date, and show us fine muds, coarse grits, 

 and volcanic accumulations intermingled one with another, and with 

 intermediate types of deposit. These form a fixed starting-point, whence 

 we may set out on our examination of the metamorphism of the region. 



3. The Alteration of the Sedimentanj Eoclcs. — Although that group of 

 rocks which on stratigraphical grounds is considered to be the oldest 

 shows the most complete metamorphism, it does not appear to be a 

 general rule that the amount of change in a rock of the series being 

 studied is proportional to its age. The amount of alteration appears, in 

 fact, to depend in part upon the constitution of the rock, and in part upon 

 accidental circumstances, amongst which latter may, perhaps, be included 

 the intrusion of vast masses of granite. The rocks which have most 

 resisted alteration have been either very fine-grained rocks, which may 

 possibly have been impervious, and those in which the fragments have 

 been so closely packed that there has been little room for matrix between. 

 The least amount of alteration of a chemical kind consists in the intro- 

 duction of a little green-mica or sericite into the original interstices, and 

 very few rocks have escaped this amount. The interest, however, arising 

 from the alteration commences when it is carried further. 



In this case we may consider separately the alterations which are of 

 chemical, and those which are of mechanical, nature. The chemical 

 alterations are those which bring about the crystallisation of the materials 

 i7i situ. With the cause of the chemical alterations itself we need have 

 nothing at present to do. The crystals arising in the rock, if they 

 possess one axis longer than the other, as is usually the case, may have 

 this axis constantly fixed throughout the rock in approximately the 

 same direction. In this case the elements are orientated ; but if there is 



