476 



TEAP DIKES. 



[Ch. XXIi 



CHAPTER XXIX. 



VOLCANIC ROCKS — Continued. 



Trap dikes — sometimes project — sometimes leave fissures vacant by decomposi- 

 tion — Branches and veins of trap — Dikes more crystalline in the centre — 

 Strata altered at or near the contact — Obliteration of organic remains — Con- 

 version of chalk into marble — Trap interposed between strata — Columnar and 

 globular structure — Relation of trappean rocks to the products of active vol- 

 canoes — Form, external structure, and origin of volcanic mountains — Craters 

 and Calderas — Sandwich Islands — Lava flowing underground— Truncation of 

 cones — Javanese calderas — Canary Islands — Structure and origin of the Cal- 

 dera of Palma — Older and newer volcanic rocks in, unconformable — Aqueous 

 conglomerate in Palma — Hypothesis of upheaval considered — Slope on which 

 stony lavas may form — Extent and nature of aqueous erosion in Palma — Island 

 of St. Paul in the Indian Ocean — Peak of Teneriffe, and ruins of older cone — 

 Madeira — Its volcanic rocks, partly of marine, and partly of subaerial origin — 

 Central axis of eruptions — Varying dip of solid lavas near the axis, and further 

 from it — Leaf-bed, and fossil laud-plants — Central valleys of Madeira not 

 crater's, or calderas. 



Having in the last chapter spoken of the composition and mineral 

 characters of volcanic rocks, I shall next describe the manner and position 

 in which they occur in the earth's crust, and their external forms. The 

 leading varieties both of the basaltic aad trachytic rocks, as well as of 

 greenstone and the rest, are found sometimes in dikes penetrating strati- 

 fied and unstratified formations, sometimes in shapeless masses protruding 

 through or overlying them, or in horizontal sheets intercalated between 

 strata. 



Volcanic or trap dikes. — Fissures have already been spoken of as oc- 

 curring in all kinds of rocks, some a few feet, others many yards in width, 

 and often filled up with earth or angular pieces of stone, or with sand and 

 pebbles. Instead of such ma- 

 terials, suppose a quantity of rig. 624. 

 melted stone to be driven or 

 injected into an open rent, and 

 there consolidated, we have then 

 a tabular mass resembling a 

 wall, and called a trap dike. 

 It is not uncommon to find such 

 dikes passing through strata of 

 soft materials, such as tufi", 

 scoriae, or shale, which, being 

 more perishable than the trap, 

 are often washed away by the 



, . , Dike in valley, near Brazen Head, Madeira, 



sea, rivers, or rain, in which (From a drawing of Capt. Basil Hail, E. N.) 



