ON THE OLDER EOCKS OF ANGLESEY. 395 



Chlorite is equally abundant as a decomposition product in the igneous 

 as in the sedimentary rocks. It is formed from either hornblende, epidote, 

 or mica. It shows feeble polarisation, and therefore is not the green- 

 mica of the sedimentary rocks. 



Sericite is also common as a result of the decomposition of white mica, 

 or as an infilling of the interstices between other minerals. 



Sphene is an abundant and characteristic element in the homblendic 

 rocks of the Central district, especially in one at Craig-yr-allor (131), 

 The crystals are never perfect, but their brownish tint, high refractive 

 index, characteristic cleavages, and weak double refraction are quite dis- 

 tinctive. It occurs in the midst of felspars or other minerals in irregular 

 grains, which are often worn so as to look as if they belonged to a pre- 

 vious generation. It is sometimes associated with ilmenite in such a way 

 as to suggest its derivation from that mineral, as has been shown to be 

 the case in some American rocks. 



Ilmenite mid Leucoxene. — The presence of one or other of these de- 

 pends on the state of decomposition only, so that the presence of the 

 latter indicates the original presence of the other. The dark black sub- 

 stance which is associated either with leucoxene or sphene is taken to be 

 ilmenite. It occurs in abundance in the hornblendic rocks of the Central 

 and Western districts, and also in the finer-grained granites. The 

 leucoxene is often in skeleton crystals, only partially presenting hexagonal 

 outlines. It is best seen in the diabase between Amlwch and Parys 

 Mountain (229). 



Rutile is not a common mineral here. It occurs as isolated crystals of 

 prismatic form in some of the hornblendic rocks of the Central district, as 

 at Plas Llanfihangel (136), and also in much smaller grains in some of 

 the finer rocks of the same kind at Gaerwen (177). 



Apatite is an abundant mineral in the hornblendic rocks. In mode of 

 occurrence it agrees entirely with the sphenes, and is seen side by side 

 with them in the rocks at Penterfyn (134). Like that mineral its refrac- 

 tive index is high, but not so high, and it is remarkable for its beautiful 

 clearness. It often occurs in characteristic hexagonal prisms. There are 

 many occurrences, however, of irregular masses, which by all other 

 characters must be assigned to the same mineral. Apatite also occurs 

 in needle-like prisms in several of the felspars and quartzes of the 

 granites. 



Garnets are numerous in certain of the rocks, especially in the volcanic 

 group north-east of Parys Mountain, both in the crystalline and the 

 associated tufis. But they are very peculiar in their characters. They 

 seldom even make an approach to a crystalline form, but sometimes they 

 are round and sometimes they spread out in a kind of ophitic sheet, 

 without regular boundaries. Moreover, there is often veiy little garnet 

 left in them. In the first place they contain many inclusions, mostly of 

 quartz, but sometimes of sphene or apatite. Then the actual garnet has 

 been cracked in many directions, and along these lines runs a band of 

 ferric dust, bounded on either side by sericite, occupying, on the whole, 

 a considerable portion of the mineral. The intervening fragments have 

 mostly turned to chlorite, and there is only left here and there a few 

 small pieces, which prove to be isotropic and highly refracting, to repre- 

 sent the original material, and even these are often broken out in the 

 cutting of the slide. 



Zircon seems rare in these rocks, unless it has sometimes been mis- 



