858 



THE WONDERS OF GEOLOGY. Lect. VIII. 



In some of the slate districts, where the trap has burst 

 through and overflowed the strata, fragments of slate are 

 found imbedded in the basalt, appearing to have been 

 detached from the rock at the intrusion of the lava, and 

 enveloped while the latter was in a state of fusion. 



Lign. 195. — Vertical channels in Sandstone strata, left by decomposed 

 trap-dikes ; at Straithaird, Isle of Sky. 



{Dr. Macculloch's Western Isles.) 



Sometimes the fractures and displacements of the strata 

 are on so small a scale as to exhibit the relative connexion 

 of the separated portions, as shown in this sketch of trap 

 intruded between sandstone, in the Isle of Arran (Lign. 196, 

 Jig. 4). This island, which is the largest in the Firth of 

 Clyde, presents, like the Isle of Wight in the south-east of 

 England, an epitome of the geology of the neighbouring 

 mainland. " The four great classes of rocks — the fossili- 

 ferous, volcanic, plutonic, and metamorphic, are there all 

 conspicuously displayed within a very small area, and with 

 their peculiar characters strongly contrasted." * 



28. Granite. — Various modifications of the compound 

 mineral termed granite, constitute a great proportion of the 

 primary rocks, and are found almost every where beneath 

 the gneiss and mica -schist, and often in contact with strata 



* Mr. Lyell's Elements, vol. ii. p. 371. I much regret that my 

 limits will not admit of a detailed notice of this most interesting 

 Island ; and I must refer the reader to the work cited, and to the 

 excellent guide to the Geology of the Isle of Arran by Mr. Ramsay 

 published in Griffiths' Scientific Miscellany, Glasgow, 1841. 



