ON THE OLDER BOCKS OF ANGLESEY. 393 



CRYSTALLINE EOCKS WITH THE HABIT OF IGNEOUS KOCKS. 



The nature and origin of the rocks of this group have been and still are 

 the subject of much difference of opinion. Those in which no foliation is 

 perceptible are considered granites and diorites by some, while others 

 have noted in them peculiarities which have suggested that they are ex- 

 tremely altered sedimentary rocks, and have made comparisons between 

 them and the Laurentian gneiss. Those in which schistosity is manifest 

 have been generally taken to be of sedimentary origin ; but recently it has 

 been shown that similar structures may characterise igneous rocks under 

 suitable circumstances. 



The solution of these questions will not depend on the microscope 

 alone, and the writer has shown what he believes to be satisfactorv 

 evidence of intrnsion, and therefore or igneous origin, in both cases. 

 However, the present business is to examine their structure, that any 

 conclusions which may be come to may be in the light of our knowledge 

 of this. One thing is certain, that if they be altered rocks we have no 

 knowledge of any unaltered representatives of them in Anglesey, and 

 can only study them as they are, It is, however, for convenience, and 

 not with the intention of begging the question, that in their description 

 they will be referred to as granites, felsites, diorites, &c. Many of these 

 rocks have undergone much alteration, even after they were crystalline 

 as we now find them ; but in this case the alterations are so dependent 

 on the original rock that they cannot be considered separately, but we 

 can only consider — (1) their component minerals ; (2) their structure, 

 original and induced. 



Minerals of the Rocks of Igneous Habit. — Quartz is far less abundant than 

 in the rocks of sedimentary origin. In one large subdivision it is absent 

 from the original rock, and occurs only in veins or bands in which it has 

 segregated. Even in the other subdivision it never forms the greater 

 part of the rock, and is more common as the latest-formed mineral in 

 the intervals between the others than as one of primary consolidation and 

 of large size. This is a common feature of the quartz of igneous rocks ; 

 on the other hand, considering the exceedingly quartzose character of the 

 undoubtedly sedimentary series, it would be rather strange, though not 

 beyond the bounds of possibility, that a more basic group should have 

 succeeded them in the same neighbourhood. 



Felspar. — In the absence of the characteristic banding of plagioclase, 

 and in the presence of abundance of saussuritic decomposition, it is not 

 always possible to determine the species, especially when there are no 

 crystal outlines or cleavages. Doubtless much is orthoclase, but plagio- 

 clastic species must be at least equally abundant. With regard to the 

 banding, nearly the same may be said as with regard to the fragments, 

 &c., in the sedimentary rocks. Perfectly regular banding, certainly repre- 

 senting twinning on the albite plan, is very common, but the lines often 

 die out in the midst of the crystal. There are seen also in the coarser 

 granites the finer cross lines, which represent the pericline twinning. 

 The appearances referred to microcline are more abundant in these rocks, 

 and are sometimes beautifully shown, as in the granite at Ynys Dodyn 

 (118). They have all the characteristic indefiniteness and changeability 

 as the slide is rotated, and are also seen cutting at oblique angles down 

 to 60°, probably owing to the obliquity of the section. It is suggestive 

 of these bands being developed by pressure that they are found chiefly in 



