ON THE OLDER ROCKS OF ANGLESEY. 409 



Anotlier totally distinct form of serpentine occnrs near Cruglas (52). 

 Quite half of it is composed of very minutely crystalline calcite or dolomite, 

 not forming a mosaic, but speckling the whole rock in clumps. These 

 clumps are in some places irregular, and in others occupy more definite 

 shapes ; the remainder, or groundmass, is of crossing fibres of serpentine 

 of various sizes and dispositions. There is also mucli black magnetite, 

 sometimes forming the centres of patches surrounded by cracks, but 

 never occurring in the cracks themselves. This calcite is doubtless an 

 infusion from neighbouring calcai-eous rocks, and has assisted in the 

 alteration of the rock. At first sight it seems impossible to recognise any 

 original minerals ; but towards the circumference of the slide are seen 

 calcitic areas which are outlined in such a way as to suggest the original 

 presence of augite ; and in one case there is a section without calcite, 

 distinguished from the rest by its transparency, and having very dis- 

 tinctly the section of a crystal of augite, cut perpendicularly to its vertical 

 axis. This remains dark between crossed Kicols, and gives a biaxial 

 cross in convergent light. In another case a crystal of the same shape 

 and properties has narrow lines of calcite along the nearly rectangular 

 directions of cleavage. These we may safely consider to be altered 

 auo-ite. In certain other areas the calcite is seen orientated in the direc- 

 tion of the longest axis of the original crystal, and the serpentine is also- 

 separated along lines parallel to this. It is probable that these areas 

 represent enstatite. It is not so easy to recognise the original olivine,, 

 but when the slide is examined with a hand-glass, there is evidence that 

 half of it is occupied by a single large crystal lying in the centre. A 

 portion with a yellower tint than the rest is outlined by a polygon ; and 

 it is always beyond this boundary that the indications of augite and 

 enstatite occur, while within, all is characteristically irregular. If the 

 nature of this be rightly judged, the original rock was a porphyritic 

 Llerzolite. It is now an ophicalcite. 



In the above rock the neighbouring calcite has affected the serpentine ; 

 in another close at hand (.53) the converse phenomenon is seen. Of this 

 the same calcitic crystalline material forms the bulk, but there is nothing 

 in any way to suggest previous crystals whose place it has taken. On the 

 contrary, the whole is traversed by irregular, undulating, subparallel lines- 

 of magnetite, from which, as also from minor cracks without magnetite, 

 the serpentine crystals grow out in radiating sheaths. This is most easily 

 interpreted as a recrystallised limestone, into which serpentinous matter 

 has been infused from without. 



5. The Isolated Masses and Di/l-es.- — For this section are reserved all 

 such peculiar rocks as are either known from field evidence to form 

 dykes, or occur in so sporadic a manner that they are probably of similar 

 origin to dykes. No dykes, however, that are intrusive into Ordovician 

 rocks, or are similar in character to such as are, will be included. For 

 all that is known to the contrary, all the following rpcks may have been 

 pi'oduced before the commencement of the Ordovician period. We may 

 commence with the felsites. 



No felsites have been recorded, or have been observed by the writer, 

 in the Eastern district, and those that occur in the Western and Central 

 districts, and that south of Traeth Dulas are so intimately associated with 

 their respective granites that they have scarcely a separate existence. 

 Thus the only isolated felsites occur in the Northern district, of which 

 four have been examined. 



