Yol. 56.] THE GEOLOGY OF NORTHERN ANGLESEY. 249 



differ widely in age, composition, and texture. They may be 

 conveniently divided into two groups, according as they were 

 injected before or after the last great period of movement which 

 affected the rocks of Anglesey ; this classification practically 

 separates them into pre-Silurian and post-Ordovician 

 intrusions. 



(a) Pre-movement Intrusions. 



Granite. — Near the western limit of the district is the granite 

 of Mynachdy, described by Prof. Blake. Another mass occurs 

 beyond the limits of the district at Pen Bryn-yr-Eglwys. As Prof. 

 Blake has pointed out, the granite is usually much crushed, and it 

 appears to be of the same character and age as the great masses of 

 Central Anglesey. 



Serpentine and its Associates. — Serpentines occur in 

 several localities in Northern Anglesey among rocks of the Green 

 Series, and in each area they are associated with a purple limestone 

 unknown elsewhere in the district : they also contain bands of ophi- 

 calcite. One mass at Tre-gele depicted on the Geological Survey 

 map is not now exposed, but Prof. Blake found the purple limestone 

 there. 1 Another mass is seen in some small quarries, near the Chapel 

 at Llanfechell. The limestone, which lies north of the serpentine, 

 is a red to reddish-purple compact rock, often veined with white 

 calcite ; it has been crushed and sheared, and iron-ores occupy the 

 gliding-planes. One specimen [N.A. 100] consists of coarse and 

 fine granular layers, much contorted and sheared together, the finer 

 calcite being full of iron-ore. It effervesces with acid, but not very 

 freely. The serpentine is usually a dull, dark green, compact fibrous 

 rock, contains chrysotile-veins, is occasionally banded, and some- 

 times brecciated. Fragments of the purple limestone are found in 

 it, and it passes into opbicalcite. The limestone is seen again 

 200 yards to the south-south-west, where it is associated with 

 ophicalcite-schist [N.A. 106J. 



Another exposure of these rocks occurs 3J miles away in a west- 

 north-westerly direction, in a small disused quarry north of Mynachdy 

 Lodge. The purple limestone which occupies the greater part of the 

 quarry is precisely identical with that of Llanfechell. The serpen- 

 tine lies mainly on the south side of the exposure, but occurs as 

 streaks and patches in the limestone. Near the southern boundary 

 of the serpentine and the schists is a dolerite-dyke, intrusive into 

 the former. 



The serpentine is seen once more 600 yards away, on the coast 

 at the western end of Porth yr Ysgraff. The intervening area is 

 covered by Drift, so that it cannot be stated whether or not the 

 rock is continuous with the last-mentioned exposure. The serpen- 

 tine has here a schistose character, the structural planes dipping in 

 a northerly direction, and varies in colour from green to yellow, 

 with frequent patches and veins of bright red. It passes down 



1 Quart. Journ. Geol. Soc. vol. xliv (1888) p. 517. 



