The highest lavas of Cader Idris, forming the ridge to the south of Llyn Cau, are separated from the amygdaloid just described by a thick zone of black slate with thin ashy intercalations, beyond which comes the coarse volcanic agglomerate already referred to as containing blocks of felsite a yard or more in diameter. These lavas are true felsites, sometimes beautifully spherulitic and exhibiting abundant flow-structure, like some of the felsites of the next or Bala volcanic period.[151] The petrography of these rocks still remains to be worked out. [Footnote 151: Messrs. Cole and Jennings, _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 430. From the examination of slices prepared from a few of the felsites of the Dolgelly district, Dr. Hatch observed a "striking difference between their characters and those of the Cambrian felsites of Caernarvonshire. The porphyritic constituent is now no longer quartz, but felspar (plagioclase), and the rocks belong, not to the rhyolitic, but rather to the less acid trachytes, perhaps even to the andesites."] The volcanic series of Cader Idris sweeps northward through the chain of Aran and Arenig, and then curves westward through the group of Manod and Moelwyn, beyond which it rapidly dies out. In its course of about 45 miles it undergoes considerable variation, as may be seen by comparing a section through Moelwyn with that through Cader Idris already given. According to the researches of Mr. Jennings and Mr. Williams,[152] the main mass of volcanic material in the northern part of the region consists of fragmentary rocks varying in texture from agglomerates into fine tuffs, but showing some differences in the succession of beds in different localities. [Footnote 152: _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 368.] The Tremadoc group of strata clearly underlies the volcanic series of these more northerly tracts. But it contains, so far as appears, no intercalation of volcanic material. The inference may thus be drawn that the eruptions began in the Cader Idris district, and did not extend into that of Manod and Moelwyn until after the beginning of the Arenig period. Above the Tremadoc group lies the well-marked and persistent band, about 13 feet thick, known as the Garth grit, which has been already referred to as a convenient base-line to the Arenig group. [Illustration: Fig. 49.--Section across the Moelwyn Range.[153] 1, Tremadoc Group; 2, Garth or Arenig grit (base of Arenig group); 3, Arenig slates, etc.; 3^1, Lower slate band; 3^2, Middle slate band; 3^3, Upper slate band; 4^1, Lower agglomerate; 4^2, Middle agglomerate; 4^3, Upper agglomerate; 5, Llandeilo group; G, Granite boss of Moel tan y Grisiau. ] [Footnote 153: After Messrs. Jennings and Williams, _Quart. Journ. Geol. Soc._ vol. xlvii. (1891), p. 371, and Horizont. Sect. Geol. Surv. Sheet 28.]