552 E. B. Taivney — Woodwardian Laboratory Notes. 



scattered throughout the ground, but none of the original horn- 

 blende is preserved. 



Hence we consider the whole mass of Carn Boduan to consist of 

 grey porphyrite. The principal divisional planes of the rock are 

 nearly vertical ; this is well seen in walking over the hill, or even 

 from below on the W. side, the vegetation finding root in the cracks 

 accentuates this feature ; these planes strike N. 25 E. — S. 25 W. 

 Notwithstanding its being opposed to Dr. Hicks's remarks \l.c. p. 299], 

 I must advocate a return to the view of the Geological Survey. 

 There is every apjDearance of the mass being an intrusive rock pro- 

 truding through Cambrian shales, which are seen lying at a steep 

 angle in the vicinity; I could not find an actual junction. Plainly 

 the rock is not metamorphic, it has the jointing and structure of an 

 igneous rock throughout. Microscopic analysis applied to pieces 

 from various parts of the hill confirms this view, so that there is 

 every appearance of its being an intrusive boss. 



I pass now to the consideration of a series of igneous masses which 

 extend with more or less interruption from Nevin to Clynnog. Most 

 of these are included by Dr. Hicks among his Pre- Cambrian series ; 

 he in the text acknowledges the partial intrusive origin of one of the 

 Yr Eifl hills \1.g. p. 299] on receiving Prof. Bonney's report on the 

 rock from Trevor quarries, but in the sketch-map they seem included 

 as Arvonian. It seems to me, however, that there is sufficient 

 evidence that these are all igneous rocks intruded through Cambrian 

 shales in the manner delineated on the map of the Greological Survey. 

 These rocks, though diversified in colour, appearance, and grain, 

 vary chiefly between granite -porphyrj'' and syeuite-poi'phyry to 

 porphyrite or diorite. They are called "felspar-porphyry often 

 syenitic" on the Survey Map (and sometimes diorite in the text), 

 which well expresses their appearance. 



To begin with the mass about one mile N.E. of Clynnog at Cil y 

 Coed; this is a quartz -felsite of pinkish-grey colour, with some 

 green patches, the quartzites more niimerous than the felspar crystals. 

 Some of the latter are seen with a hand lens to have triclinic twinning. 

 It is of a different type of rock to the following. 



Bwlch Mawr, a hill S. of Clynnog, from the N. side of the hill 

 below the summit. — The ground is dark grey, and looks fine-grained 

 to the eye ; in it are scattered numerous whitish felspars averaging 

 about Y inch in diameter. Even a hand lens shows that many are 

 plagioclase. 



The microscope confirms the fact that many of the larger felspars 

 are plagioclase, and from the predominance of crystals with a small 

 extinction angle, they should belong to the oligoclase group. The 

 ground was apparently microcrystalline, but it is now so decomposed 

 that not much is to be made out. Under crossed Nicols there are 

 clearer spaces of doubly refracting matter rendered turbid by fine 

 dust, and darker granulated interstitial matter which seems almost 

 optically inert ; a few nests of quartz seem due to secondary action. 

 Magnified only fifty diameters, part of the ground is found heaped 

 together to form dirty grey granulated more or less opaque areas, 



