254 



MR. C. A. MATLET ON THE 



[May 1900, 



basic material by the occurrence of a piece of basalt clinging to the 

 side of a felsite-dyke which lies a short distance south of Cemaes 

 Pier. Basalt was found again in contact with felsite north of the 

 pier, and towards low-water mark a third felsite-dyke was discovered 

 which had a narrow basalt-dyke on each side of it. The basic 

 material seemed to be intrusive into the acid. Examples of these 

 composite dykes have since been observed in various parts of the 

 northern district of Anglesey. 



In a field near the road south of Groes-fechan is a felsite-dyke, 

 about 20 feet wide, easily discernible by its white crust. Clinging 

 closely to its northern boundary, and sometimes forming small bays 

 in it, may be noticed patches of basalt. Midway between this 

 dyke and Cefn-coch Factory is a rounded rocky knoll in a field : 

 its sides are evidently basic, but the centre of the mass is a felsite. 



A good example of a composite dyke is seen near the serpentine 

 on the road from Llanfechell Chapel to Llyn Geirian. An exca- 

 vation has been made, in a small garden by the roadside, into one 



of these dykes. 



Fig. 7. — Section in the composite dyle, west of 

 Llanfechell. 



lickensicks 



si ic ken sides 



The relation- 

 ship of the two 

 magmas is 



shown in fig. 7 ; 

 the basic, which 

 is distinctly 

 slickensides intrusive into 



the acid portion 

 (a granophyre), 

 has worked its 

 way along joints 

 and cracks, and 

 has caught up 

 1 fragments of 



= Basalt. th . e acid rock - 



Slickensides oc- 



about 16 feet.] cur on both 



acid and basic 

 parts. This is the only acid dyke that I have found which shows 

 complete invasion by the basic ; in the other cases the basic material 

 has been merely ' insinuated ' along the edges of the dyke. 



One more example may be cited. At Hafod-onen, south-west 

 of Amlwch, is a quarry in a beautiful granophyre described by 

 Prof. Blake. 1 This forms a boss rather than a dyke. It has a width 

 of 24 feet, and between its southern face and the country-rock is a 

 2-foot dyke of basalt. On its northern side it is seen again at 

 intervals to be in contact with a basaltic rock. The junction of the 

 latter is not very straight, nor does it always touch the granophyre. 



The dykes south of Llanfechell have not been traced far along 



1 = Granophyre ; 2 

 [Width of the part shown = 



1 Eep. Brit. Assoc. 1888 (Bath) p. 410; see 

 of microscope-section N.A. 109. 



the description on p. 253 



