114 BASALTIC AND VOLCANIC ROCKS. 
Supermedial 
Order, 
Medial Or- 
der. 
Submedial 
Order. 
/Chalk 
I Green Sand 
1 Iron Sand 
J Upper Oolite 
^ Middle Oolite 
1 Lower Oolite 
1 New Red Sandstone 
/Millstone Grit . 
{ Coal Measures . 
Carboniferous Limestone . 
I Do. Sandstone and Graywacke 
/ Transition Limestone 
< Do. Slate . 
reet. 
700 
600 
950 
770 , 
880 
820 
2100 
7720 
700 
1000 
850 
10,000 
12,550 
1800 
, 2500 
Total 
4300 
24,350 
CHAPTER XL 
BASALTIC AND VOLCANIC ROCKS. 
Volcanic Formations, — Trap Rocks. — Mineral Composition.— 
Augite Rocks. — Basalt. — Gieenstone. — Trachyte.— Clink- 
stone. — Porphyry. — Amygdaloid.— Lava.--Scoriae. — Pumice. 
— TufF. — Conglomerates. — Wacke. — Whinstone. — Pitch- 
stone. — Volcano of Kirauea. — Trap Dikes. — Fingal's Cave.— 
Staffa.— Rocks altered by Dikes. — Faults. — How they cause 
a Dislocation of Strata. 
Volcanic formations constitute some of the most 
interesting portions of the globe for the geologist 
to contemplate ; for here he sees the effects of those 
causes still in operation, which, doubtless, have had 
a powerful influence in consolidating, as well as 
forming, the earth's surface in the most remote 
geological epochs. 
The rocks which are recognised as belonging to 
the present class have been, called trap rocks, from 
