§ 24. 



BASALT, OR TRAP. 849 



ages ; it also occurs in layers spread over the surface of 

 the strata, or interposed between them. Many modern 

 lavas differ so little from basalt, that it is unnecessary to 

 adduce proof of the volcanic nature of this rock. Dr. Mac- 

 culloch observes, that from lava to basalt, and from thence 

 to syenite, porphyry, and granite, there is an uninterrupted 

 succession : as agents in geological changes trap and granite 

 are identical; and that it is a mere dispute about terms to 

 refuse the name of submarine lavas to basaltic dikes. " They 

 are as much the product of extinguished volcanoes, although 

 they do not now emit fire or smoke, as are those of Italy, 

 where the volcanic action has ceased."* Beds of basalt, of 

 a friable and coarse texture, are often found in the older 

 rocks ; these are volcanic ashes and grits, that have been 

 formed at the bottom of the sea, during the accumulation of 

 the sedimentary matter with which they are associated 

 {ante, p. 779). In some places they appear as currents or 

 sheets of pure volcanic materials ; at others they envelope 

 marine remains, pebbles, sand, and fragments of rocks: 

 some layers consist of v fine volcanic scorias passing into sand ; 

 and all these varieties alternate with beds composed ex- 

 clusively of shelly and marine sediments, so that no doubt 

 can be entertained that the diversified masses thus arranged 

 in parallel strata, must have been formed during the same 

 period of igneous action. These evidences of ancient 

 volcanic operations are similar to those observable in the 

 modern deposits of Sicily, where banks of existing species 

 of marine shells, now at considerable heights above the sea, 

 are so intercalated with volcanic matter, that no other 

 inference can be drawn than that the whole were of con- 

 temporaneous marine formation, f 



The most remarkable form assumed by basalt, is that of 

 regular pillars, or columns, clustered together ; a character 

 also observable in some recent lavas ; the columnar basalts 



* System of Geology, vol. ii. p. 100. f Silurian System, p. 75. 



