420 Mineralogical Olservations. [Dec. 



Appearances of tills kind occur in the granite of Braemar, of Aber- 

 deen, and other districts in Scotland * 



2. In gneiss districts we occasionally observe masses of granite in 

 the gneiss, from a few Inches to many fathoms in extent. These 

 vary in shape, being round, oval, sharp-angular, blunt-angular, or 

 with long arms shooting from them into the bounding rock in the 

 form of veins. They are often observed passing into the bounding 

 gneiss ; a proof of their being of cotemporaneous formation with 

 it. Interesting appearances of this kind occur in the bold rocky 

 coast which extends from Aberdeen to Finen. In the country 

 immediately above this coast we meet with hillocks of granite rising 

 through the gneiss ; these are cotemporaneous masses of granite in 

 the gneiss, and not portions of the older or central granite, f 



3. In districts composed of syenite we frequently observe im- 

 bedded portions of porphyry and trap. These vary in size, from a 

 few inches to many fathoms, and pass gradually into the bounding 

 syenite ; thus proving that they are of cotemporaneous formation 

 with it. The rocky coasts around Peterhead, the syenite of Glen 

 Tilt, and the syenite hills of Galloway, afford fine examples of 

 these appearances. It is of consequence to attend to the distinction 

 here pointed out, otherwise we run the risk of confounding together 

 in our descriptions the syenite, porphyry, and trap formations, and 

 might be led to describe a country as composed of three formations, 

 when in reality it contained but one. Such mistakes can scarcely 

 be committed when we have a coast to describe where the appear- 

 ances are almost always well exposed ; but in the interior, where 

 the rocks are much hid, the error has been fallen into. 



4. In mountains of coarse granular granite we frequently meet 

 with variously shaped imbedded portions of small granular granite. 

 These at first sight might be confounded with fragments ; but the 

 fact of many of them passing into the bounding rock shows that 

 they are cotemporaneous formations. It is worthy of remark, that 

 these imbedded portions have sometimes a globular or oval form, 

 but a slatv structure ; and that, when many of them occur together, 

 an appearance is formed precisely like that exhibited by topaz rock ; 

 that is to say, the mass is granular in the large, and slaty in the 

 small. X 



5. Masses of lime-stone of considerable purity occur in some 

 districts imbedded in calcareous sand-stone ; these are sometimes of 

 considerable magnitude j and being in general more compact than 



* It is probable that some bills of gneiss, porphyry, &r. in granite districts, 

 are cotemporaneous masses imbedded in the granite, and portions of the gneiss or 

 .porpinry formations. 



t. We sometimes meet with veins of compact gneiss or of grange so dark 

 coloured that they might be confounded with basalf, or si me rock of the trap 

 series. I suspect some of the trap veins described as occurring in granit- arc of 

 this nature. 



' * The topaz nek may b<» mentioned a- an cxpmplc of a rock formed by crys- 

 tallization, yet haw og a striking brecciated. of conglomerated aspect. 



I 



