PART I. CHAPTER VIII. 



103 



Volcanic Dikes. 



CHAPTER VIII. 



VOLCANIC ROCKS — Continued. 



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

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

 Foreign fragments of rock imbedded — Strata altered at or near the contact — Ob- 

 iiteration of organic remains — Conversion of chalk into marble — and of coal into 

 .coke — Inequality in the modifying influence of dikes — Trap interposed between 

 strata — Columnar and globular structure— Relation of trappean rocks to the pro- 

 ducts of active volcanos — Submarine lava and ejected matter corresponds gene- 

 rally to ancient trap. 



Having in the last chapter spoken of the composition and min- 

 eral characters of volcanic rocks, I shall next describe the man- 

 ner and position in which they occur in the earth's crust, and 

 their external forms. Now the leading varieties, such as basalt, 

 greenstone, trachyte, porphyry, and the rest are found some- 

 times in dikes penetrating stratified and unstratified formations, 

 sometimes in shapeless masses protruding through or overlying 

 them, or in horizontal sheets intercalated between strata. 



Volcanic dikes. — 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 quantity of melted stone to be driven or injected into 

 an open rent, and' there consolidated, we have then a tabular 



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

 (See the annexed figure.) 



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



Fig. 89. 



.-.1' >C"V^~ 



mass resembling a 

 wall, and called a 

 trap dike. It is not 

 uncommon to fmd 

 such dikes passing 

 through strata of soft 

 materials, such as tuff 

 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 



