114 BASALTIC AND VOLCANIC ROCKS. 



( Chalk 

 I Green Sand 

 | Iron Sand 

 Supermedial j Upper Oolite 

 Order. } Middle Oolite 

 I Lower Oolite 

 I New Red Sandstone 

 V 



Medial Or- 

 der. 



Submedial 

 Order. 



( Millstone Grit 

 J Coal Measures . 

 { Carboniferous Limestone . 

 j Do. Sandstone and Graywacke 



( Transition Limestone 

 < Do. Slate .... 



Feet. 

 700 

 500 

 950 

 770 

 880 

 820 

 2100 



7720 



700 

 1000 



850 

 10,000 

 V, 



, 1800 

 , 2500 



-12,550 



4300 



Total 



24,350 



CHAPTER XL 



BASALTIC AND VOLCANIC ROCKS. 



Volcanic Formations. — Trap Rocks.— Mineral Composition.— 

 Augite Rocks. — Basalt. — Greenstone. — 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 



