250 ME. C. A. MATLEY ON THE [May I9OO, 



into a pale calcareous rock (with tremolite ? developed in it) which 

 curves round and bends over a mass of dark rock : this, in part at 

 least, is a hornblendite made up of hornblende, ilmenite, and tre- 

 molite or actinolite. 



By analogy with those of the Holyhead district these serpentines 

 should be regarded as altered igneous intrusions, but such an origin 

 cannot be affirmed with any certainty, as they show no igneous 

 structure. Their sporadic occurrence is evidence of their being 

 of intrusive nature, but this argument applies with equal force 

 to the purple limestone, which among many square miles of the 

 green phyllites and slates of the northern district is only known in 

 three isolated localities, and then always in connexion with the 

 serpentine. 



Whatever the origin of these serpentines and limestones, both are 

 clearly of earlier date than the great movements that affected the 

 area, and the limestone cannot therefore be looked upon as a product 

 of infiltration from any post-Ordovician limestone which may once 

 have overlain this area. 



The following notes, kindly supplied by Prof. Watts, are descrip- 

 tive of some of these rocks : — 



[N.A. 115; near Llanfechell Chapel.] Ophicalcite. Masses of 

 granular calcite and serpentine mixed together. The calcite contains 

 one broken crystal of picotite. The serpentine occurs in colourless, 

 brightly-polarizing crystals, set in an almost isotropic, dull-green 

 serpentine. From these patches the blades penetrate into the 

 surrounding calcite and into and between the crystals and grains of 

 it in the same way as the silica in N.A. 123 (see p. 244). Magnetite 

 is present as usual. 



[N.A. 19 ; same locality.] Ophicalcite. Streaks and large patches 

 of serpentine set among grains and often rhombohedra of calcite 

 (or dolomite). Smaller patches of serpentine, generally associated 

 with more coarsely crystalline calcite, and often with aggregates of 

 magnetite. Soir.e of these patches consist of calcite and magnetite 

 alone. 



[N.A. 106; south-south-west of the above.] Ophicalcite-schist. 

 Fine intermixture of serpentine and calcite, the latter in small 

 grains set in a felt of serpentine ; minute ' augen-structure.' 



[N.A. 78; north of Mynachdy Lodge.] Coarse and fine granular 

 limestone contorted and mixed together ; some folia filled with 

 colourless serpentine ; much interstitial muscovite. 



[N.A. 102; Porth yr Ysgraff.] Foliated serpentine. One bit of 

 picotite. Some bands rather more coarsely crystalline than the 

 rest. 



Basaltic Dykes. — Two dykes, crushed and broken by earth- 

 movements, were brought to notice in my previous paper 1 ; they 

 occur in the crush-zones of the Northern Complex. A third dyke 

 of similar appearance has been found on the coast west of Penrhyn. 



1 Quart. Journ. Geol. Soc. vol. lv (1899) p. 666. 



