102 GEOLOGY OF ARRAN. 



at once to recognize them amid inetamorphic slates ; the 

 columnar and jointed forms which these rocks so often put 

 on being also dependent upon it. All our basalts, green- 

 stones, and felstones exhibit this columnar and articulated 

 structure; but the concave and convex jointing is seen only 

 in basaltic pillars. It is plain that dikes are prismatic 

 across, because the heat passed off perpendicularly to the 

 cooling surfaces. 



52. There are several varieties of felspar distinguished by 

 their forms and other characters, as well as by their chemical 

 composition. The common dull white felspar of the coarse 

 granite is orthoclase and a potash-felspar; a much whiter 

 kind, with smaller crystal and a jagged termination is albite, 

 a soda-felspar ; another variety is oligoclase or andesite, 

 a soda-lime felspar ; labradorite (with fine play of colours), 

 found in Skye, a lime-soda felspar, the properties being 

 reversed'; and lastly, anorthite or lime-felspar. Glassy 

 felspar in large clear crystals is usually present in the Arran 

 porphyries, as those of Drumadoon and the south-west coast. 

 The first three varieties only are those usually associated with 

 quartz in rock masses. 



The felspathic rocks those containing felspar alone are, 

 viewed chemically, silicates of alumina, mixed with silicates 

 of potash or soda, lime, iron, and manganese; when horn- 

 blende or augite are in combination with the felspar, they are 

 silicates of magnesia, combined with the other silicates as 

 above. 



The felstones of Arran are, felspar porphyry, claystone, and 

 clinkstone. When a rock consists of any simple base with 

 crystals of felspar imbedded, it is called a porphyry, and is 

 named after the nature of the 1 base. Thus felspar crystals 

 being imbedded in a uniform felspar base, the rock is felspar 

 porphyry; so also we have pitchstone porphyry, hornstone 

 porphyry, and so on. But the term is often used loosely, 

 as when the base is not simple, and the imbedded crystals not 

 felspar. Compact felspar, claystone, and clinkstone are mere 



