CHAP. XII 
ARE NIG ER UPTIONS—MERIONE THSHIRE 
185 
about 45 miles it undergoes considei’able variation, as may l)e seen by 
comparing a section through Moelwyn with that through Cader Tdris already 
given. According to the researches of Mr. Jennings and Mr. Williams,^ the 
main mass of volcanic material in the northern part of tlie 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. 
The Tremadoc group of strata clearly underlies the volcanic series of 
these more northerly tracts. But it contains, so far as appears, no intercala- 
tion 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 Gartli grit, which has been already referred to 
as a convenient base-line to the Arenig group. 
In this northern district, among the sediments which overlie the Garth 
grit, layers of fine tuff begin to make their appearance, which north of 
Cwm Orthin thicken out into a considerable mass between the grit and the 
Nw Moelwyn Se 
Fig. 49. — Section across tlie Moelwyn Range. ^ 
1, Tremadoc Group ; 2, Garth or Arenig grit (base of Arenig group) ; 3, Arenig slates, etc. ; 3^. Lower slate band ; 
3^, Middle slate ban<l ; 33, Upper slate band; 4’, Lower agglomerate; 42, Middle agglomerate; 43, Upper 
agglomerate ; 5, Llandeilo group ; G, Granite boss of Moel tan y Grisiau. 
lowest of the great agglomerates. These tuffs, which mark the beginning of 
the volcanic eruptions of the district, are followed by a band of slate which 
in some places has yielded a Lingula, Orthis Oarausii, and a Tetragraptus, 
and points to an interval of quiescence in the volcanic history. We now enter 
upon an enormous thickness of agglomerates and tuffs separated by several 
bands of slate. Taking advantage of the slaty intercalations, Messrs. 
Jennings and Williams have divided this great accumulation of fragmentary 
^'olcanic material into three beds (Fig. 49). Tlie matrix of the agglomer- 
ates is compact and pale, so as to resemble and to have been called “ felstone,” 
but showing its fragmentary nature on weathered surfaces. The blocks 
imbedded in this paste range up to sometimes as much as 1 1 feet in length 
by 4 feet in width. Their minute petrographical characters have not been 
studied, but the blocks are stated to consist for the most part of “ slaty 
and schistose fragments mixed with rounded pebbles of fine-grained 
‘ felstone.’ ” They are heaped together as in true agglomerates. In the 
’ Quart. Jmmi. Geol. Soc. xlvii. (1891), p. 368. 
- After ilessrs. .Temiings and Williams, Quart. Journ. (teol. Soc. vol. xlvii. (1891), p. 371, 
and Horizon!. Sect. Geol. Snrv. Sheet 28. 
