and Voleanic Substances. 225 
through a layer of sandstone three feet thick at Birch hill 
Colliery in Staffordshire. 3 Geo. trans. 257. 
At Tividale, the coal is completely charred under the 
Rowley Rag, which there, (‘Tiviotdale near Derby, Shrop- 
shire,) is thirty-five feet thick. 
In the third place, I -have to shew that Basdie like’ Bava 
graduates into, “and is accompanied by Porphyries one 
Amygdaloids. 
Dhave already: observed after Humbold, that ZEtna is a 
porphyritic rock, surmounted by columnar basalt. Iknow 
the ambiguity that attends the name Porphyry: but I trust 
that my geological reader will acknowledge, that porphy-— 
ritic lava, and: fleetz trap porphyries are in general suffi- 
ciently distinct from primitive trap porphyries.” If 1 saw’a 
porphyritic stone crystallized throughout, I should refer it 
to the primitive porphyries ; but the confused and rounded 
specks that porphyrize a basaltic or volcanic stone, are easi- 
ly recognized:» The volcanic and basaltic porphyries are 
too similar to each other, and too dissimilar to primitive por-. 
phyries, to occasion frequent mistakes to an experienced 
eye. 
adit the porphyries concerning which I speak, so far” 
as I can collect from the authorities, shall be basaltic por- 
phyries only. Ihave well considered Humbold’s fourfold 
division of Porphyries ; ; his second class alternates with ba-. 
salt, pitch stone and obsidian ; his third class includes the 
-amygdaloids of Oberstein, which I should certainly ascribe 
to volcanic origin 5 and his fourth class consists of the fleet 
trap porphyries ‘now under consideration. 
-Porphyries of this class, alternate with’ decidedly voléd! 
nic products in voleanic districts, even: according to Jame- 
son. 3 Geognos. 192, and Humbold says it forms the sum- 
mit’ of many of. ‘the ‘volcanic mountains of ‘the Andes. 
1 Pers. Narr. 115—218. 
‘Porphyry and Araygdaloid: accompany basalt. E Geol.” 
trans. part 1, p. 277. . 
Basalt passing: into pee casete and porphyry slate. Kida. 
136. ; 
The general range of the Andes, which are fifty ers 
according to Humbold, granite, gneiss; mica slate, and clay 
slate ; but these are seen surmounted by Le ia aco. ‘Bake- 
well’s Geo. 65. 
