370 HEPORT — 1888. 



the Eastern and Norfhern districts ; occurring in the former in the lami- 

 nated schists often imbedded in chlorite, as at Hafodty (145), and also 

 as rounded derivative grains in the newer dust rocks, which is its 

 principal mode of occurrence in the sedimentary rocks of the north. 



Kaolin is perhaps the best name to apply to the white amorphous dust 

 which is so abundantly scattered over the great majority pf rocks, lying 

 either in isolated grains in the less completely crystallised bands, or 

 pushed aside into the interstices between well-formed crystals. In the 

 former case it may be pretty nearly in its original position of deposit ; 

 in the latter, it is either the residuum which has refused to crystallise, or 

 the relics of former crystalline substances which have since disintegrated. 

 There is, however, no evidence as to its chemical composition, though 

 kaolinite in definite crystalline forms has been discovered some time ago 

 in the slaty rocks of the Northern district. 



Pyrites occurs in well-formed, usually cubic crystals, in rocks of very 

 fine grain in the Northern and Central districts. It does not seem very 

 certain whether these are derivative from older rooks or authigenetic, 

 though the latter seems more probable. 



Ferrite is sometimes seen surrounding the pyrites as a red translucent 

 decomposition product ; but the substance usually quoted by this name 

 consists of a brown amorphous dust found in similar circumstances to 

 those of kaolin, but in more basic rocks. 



Garnet occurs in rounded, worn-looking, or imperfectly formed crys- 

 tals in the mica schists of the Eastern district, as near "Gaerwen (156). 

 They are here so abundant that in spite of their numerous cracks, filled 

 with a later crystallisation, there seems no evidence of their being deri- 

 vative, the whole rock being completely crystallised. These are generally 

 of a pale green tint. 



Zircons, in characteristic short prismatic crystals with worn pyramidal 

 ends, are tolerably abundant in some of the quartzites, as at Porth-yr- 

 Ogof (2). Here they are certainly derivative. They also occur in the 

 mica schist of Abersant (19) and in the quartzitic rocks of the South 

 Stack series (64, 65) ; also in the quartzose gneisses and quartzites of the 

 Central district, as at Gwalchmai (70) and Bodafon (109) ; but they have 

 not been observed in the sedimentai-y series either of the Eastern or 

 Northern districts. Possibly these latter were derived from a different 

 source. 



Such are the principal minerals which enter into the composition of 

 the oldest stratified rocks of Anglesey. Others, such as sphene, rutile, 

 apatite, and tourmaline, are either very rare, occur only as enclosures, or 

 are doubtfully determined. 



2. TJie Structure of the Sedimentary Eocls. — The present structure of 

 the rocks depends in part on their original constitution, and in part on 

 the alterations which they have undergone ; but it is not always easy to 

 determine which of these has most influence on the observed results. 

 Some of the rocks are crystalline, and are undoubtedly much altered ; 

 others are composed of dust or fragments, and these may be either origi- 

 nal or the results of crushing in situ. In this case their probable origin 

 must be sought in their stratigraphy ; and by examining the structure of 

 those rocks which are thus proved to be original, and comparing it with 

 those in which crushing may have had a considerable effect, we may 

 obtain characteristics which shall be of avail when stratigraphy gives 

 doubtful information. So fai-, then, as may be judge d by their strati- 



