MIDDLE DISTRICT OF SCOTLAND. 625 



rieties of appearance as are sometimes exhibited 

 in hand specimens. Cotemporaneous veins of a 

 granitous rock, of the same general character with 

 the sandstone, sometimes traverse it ; in other 

 instances, cotemporaneous veins of quartz, or of 

 felspar, also occur in the sandstone *. 



When the basis of the sandstone increases in 

 quantity, the sandstone gradually passes into slate- 

 clay, or into iron-clay, quartz, or limestone, accord- 

 ing to the nature of the basis or ground. The iron- 

 elay is often amygdaloidal, so that there is then a 

 transition from the sandstone into amygdaloid. 

 Many veins of sandstone are to be observed shoot- 

 ing from the sandstone rock into the amygdaloid 

 and other similar rocks, and portions of sandstone 

 imbedded in beds of trap and porphyry, send out 

 veins or branches from all sides. In other in- 

 stances, portions of amygdaloid are imbedded in 

 the sandstone, and either intermixed with the 

 sandstone at the line of junction, and gradually 

 pass into it, or they send out branches into the 

 inclosing sandstone. But this is not all the varie- 

 ty of appearances presented by the sandstone. 



* Rocks having the same structure and general appearance 

 as red sandstone, appear occasionally in transition, and also in 

 primitive districts, as in countries abounding in clay-slate and 

 gneiss, — -a fact which at first sight appears inconsistent with the 

 general distribution and arrangement of the different rock for* 

 piations. 



