and Volcanic 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 Basalt like Lava, 
graduates into, and is accompanied by Porphyries and 
Amyegdaloids. 
I have already observed after Humbold, that Etna is a 
porphyritic rock, surmounted by soli scot I know 
to the primitive es la ; but the c onfused and ro unded 
specks that porphyrize a basaltic or Selon stone, are easi- 
ly recognized. ‘The volcanic and basaltic porphyries are 
too similar to each other, and too dissimilar to primitive por- 
Bepeie’, to occasion frequent mistakes to an experienced 
Heuce 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 fourfol 
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 ; and his fourth arc sree of the fleetz 
trap porphyries now under considera 
Porphyries of this class, alternate with decidedly volca- 
ni¢ 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 Amygdaloid accompany basalt. 5 Geoi. 
trans. part 1, p- 27 277. 
18. t passing into greenstone, and porphyry slate. Kidd. 
general range ofthe Andes, which are fifty volcanic 
ie to Humbold, granite, gneiss, mica slate, and clay 
slate ; but these are seen surmounted by porphyry. Bake- 
well’s Geo. 65. 
