ON THE OLDER ROCKS OF ANGLESEY. o i 9 



Another in tlie district south of Traeth Dulas (140) is still more like a 

 felsite. Others at Porth-ceryg-defaid (83) and north of Llangwllog (87) 

 are so broken up that their origin is still more obscure. 



"We have next to study the introduction of orientation into this type 

 of rock. 'Now orientation, as shown in the altered rocks of Anglesey, 

 may be of several kinds. In a great number of instances it depends 

 solely on the occurrence of flaky minerals. The lowest kind of orienta- 

 tion is when these flakes are scattered promiscuously amongst the other 

 elements ; but, wherever they may be, their long axes lie in the same 

 direction. This may be called quincuncial orientation (see fig. 5). A 

 higher kind is when such flaky minerals occur in definite lines. In this 

 case there is often no perfect parallelism of the individuals, but they may 

 make considerable angles with each other ; yet the general direction is 

 constant. It is this kind of orientation which produces the most perfect 

 schists. It may be called linear orientation (see fig. 6). The highest 

 kind is when the orientation depends not only on special minerals, but on 

 all the elements, whose long axes are either accurately or generally in 

 the same direction. This may be called e^eme«<«Z orientation (see fig. 7). 

 In all these cases individual crystals are orientated ; but there are other 

 kinds of orientation seen only in the rock as a whole. Thus in some the 

 elements of quartz and felspar, of which the rock is composed, though 

 they may themselves be isodiaraetric, lie in bands side by side. From the 

 similarity of the result to lamination we may call this laminar orienta- 

 tion (see fig. 8). In other cases no orientation can be observed in the 

 small portion of the rock which comes under the microscope at one time ; 

 but when the slide is viewed with a hand-lens, there is seen to be a rough 

 parallelism in the collocation of the more conspicuous elements. Such 

 may be called confxised orientation. Examples of all these forms of orien- 

 tation will be found in the various rocks to be yet described. 



False orientation, which does not atfect the substance of the rock 

 itself, is that produced by a series of parallel cracks (see fig. 9), which 

 may be afterwards cemented by mineral matter, as we have seen to be 

 the case with the rocks at Point JElianus. There is also the orientation 

 due to oricrinal beddin? in the case of the laminated rocks. A very 

 remarkable instance of this may be here mentioned as occurrmg m a 

 rock at Ty-garw, near Beaumaris (195). Between crossed Nicols, when 

 the crystalline elements are most conspicuous, little or no orientation can 

 be seen, but the rock looks like an ordinary mosaic ; but with the para- 

 boloid it is seen to be broken up into fragments, which lie rather 

 irregularly in the midst of a cementing material of clear quartz with 

 larger elements, but each fragment is seen to belong to a rock which 

 wa.s originally laminated with exceedingly fine lines of ferruginous dust. 



The orientation developed in the fine-grained rocks seldom rises above 

 the quincuncial, the orientating mineral being in every case a green mica, 

 though where this is crowded in bands the orientation becomes practically 

 linear. The rocks which best exhibit this are found in the north-eastern 

 portion of the Western district and the neighbouring locality Mynydd 

 Mechell. In the rock from the latter (212) the direction of the indi- 

 vidual crystals follows the banding, as marked by the greater or less 

 abundance of the mica, and the bands follow the contortions. Hence we 

 may assume the orientation to be coincident with the bedding. But in the 

 fine schists found at Caer Ceiliog(9) and Llanddeussant (20) (see fig. 10) 

 it is diflferent. In these, and particularly in the former, certain narrow 



