Ch. XXIX.] TRAP DIKES. 609 



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 Islauds — 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 land-plants — Central valleys of Madeira not craters, 

 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 and trachytic 

 rocks, as well as of greenstone and the rest, are found sometimes in 

 dikes penetrating stratified and unstratified formations^ sometimes in 

 shapeless masses protruding through or overlying them, or in hori- 

 zontal sheets intercalated between strata. 



Volcanic or Trap Bikes. — Fissures have already been spoken of as 

 occurring 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 materials, suppose a quan- 

 tity of 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 pass- 

 ing through strata of soft materials, such as tuff, scoriae, or shale, 

 which, being more perishable than the trap, are often washed away 

 by the sea, rivers, or rain, in which case the dike stands prominently 

 out in the face of precipices, or on the level surface of a country (see 

 fig. 677). 



In the islands of Arran and Skye, and in other parts of Scotland, 

 where sandstone, conglomerate, and other hard rocks are traversed by 

 39 



