215 



great Carnarvon chain, prolonged, in fine serrated ridges, to the 

 neighbourhood of Great Ormeshead. In the low country east of 

 the Menai are slate rocks alternating with trappean conglomerates, 

 and with great masses of porphyry ranging very nearly with the beds 

 of slate. One of the larger masses of porphyry appears to have 

 been protruded after the deposition of the slates : other masses are 

 contemporaneous with the slates. To the east of the largest elon- 

 gated mass of porphyry, which strikes with the beds for about fif- 

 teen miles, commences a system of undulations affecting the whole 

 chain, the anticlinal and synclinal lines ranging very exactly with 

 the strike of the beds, i. e. N.N.E. or N.E. by N. The same pre- 

 vailing strike and the same undulations are continued into Merio- 

 nethshire ; but in the southern parts of that county the strike de- 

 viates to the north-east and south-west. One great anticlinal line 

 is traced from the county about three miles east of Festiniog to a 

 point on the coast a little south of Barmouth. Beyond this line is 

 an ascending section (very little interrupted by undulations), as far 

 as the Bala limestone. These facts are described by the author in 

 detail, and are illustrated by parallel sections at right angles to the 

 mean strike of the country. 



The rocks occupying the region are chiefly composed of felstone, 

 (compact felspar) and felstone porphyry, trappean conglomerates, 

 plu tonic silt (exactly like chloritic varieties of German Schaalstein), 

 and other erupted or recomposed igneous products : and the above- 

 named rocks alternate indefinitely with fine masses of roofing-slate, 

 and with great masses of greywacke ; and with greywacke slate, 

 often calcareous, but rarely containing beds and masses of limestone. 

 Three or four subordinate masses of such limestone are found near 

 Great Arenig, and one or two on the flanks of Cader Idris. The 

 author describes some sections where the igneous rocks predominate 

 over the aqueous ; others in which the aqueous almost exclude the 

 igneous : but the two classes of rock are so interlaced that they 

 cannot be separated, and are regarded as of contemporaneous origin. 

 Among them are however masses of greenstone (sometimes syenitic, 

 and more rarely basaltic), and other trappean masses among the 

 slates which are considered of a later date. 



Nearly all the slate rocks are affected by a cleavage, which often 

 obliterates all traces of stratification, and very seldom coincides with 

 the true beds. In the fine quarries of NantFrancon and Llanberris, 

 the cleavage planes (as in the great quarries of Cumberland) strike 

 exactly with the beds, but are inclined at a greater angle. The 

 strike and inclination of these planes is not however governed by 

 any fixed law ; for cases are pointed out where cleavage planes are 

 much less inclined than the beds, and others in which these planes, 

 even in fine quarries, deviate one or two points from the strike of 

 the true beds. Five cases are exhibited in the sections where the 

 cleavage planes continue their strike and dip through all the con- 

 tortions of the beds ; and a few cases are given of a good second 

 cleavage plane. From all the facts the author concludes that clea- 

 vage planes are true crystalline phsenomena, produced by the mutual 



