1814.] Mineraluoical Observations. 423 



are to be seen in the gneiss and granite of Aberdeenshire. Similar 

 phenomena occur in floetz districts : thus we occasionally meet with 

 beds, imbedded masses, and cotemporaneous veins, of amydgdaloid 

 or trap tuff in a mass of sand-stone of no very great extent, and 

 conversely similar repositories of sand-stone occur in masses of tuff 

 or amygdaloid. In such cases the solution appears to have con- 

 tained at the same time all the ingredients of which the different 

 rocks are composed, and these have been separated from their 

 menstruum into the form they now possess by specific attractions 

 taking place amongst the particles. Were we inclined to venture 

 farther in the hazardous field of geological speculation, we might 

 conjecture that cotemporaneous formations taking place still more 

 on the great scale might have given rise to the conical and tabular- 

 shaped hills of certain floetz formations, and that the figures of 

 many hills composed of rocks formed from a state of solution depend 

 in some degree on their mode of crystallization. 



III. — On the Intermixture of Beds and Strata at their Junctions. 



The opinion held by some observers that strata and beds are 

 seldom intermixed at their line of junction, and that transitions 

 from one bed into another very rarely occur, cannot be considered 

 correct, as will appear from the following statement : — 



1. Beds of granite in gneiss are sometimes distinctly separated 

 from the bounding rock ; in other instances an uninterrupted tran- 

 sition is to be observed from the granite into the gneiss, or the 

 granite is interrupted with the gneiss at their line of junction. 

 Similar appearances are to be observed in the beds of gneiss that 

 occur in granite. 



2. Beds of granular quartz, much resembling sand-stone, some- 

 times alternate with granite : these beds are either distinctly sepa- 

 rated from the granite at their line of junction, or they are inter- 

 mixed, and veins of the granite shoot into the quartz, and veins of 

 the quartz into the granite. Alternations of granite and quartz 

 occur in the district of Bracmar, in Aberdeenshire. 



3. Beds of green-stone frequently occur in clay-slate : sometimes 

 they are distinctly separated from the clay-slate at their line of 

 junction with it ; in other instances the two rocks are intermixed at 

 their line of junction ; and occasionally a gradual transition is to be 

 observed from the green-stone into the clay-slate. 



4. Beds of hornblende-rock and hornblende-slate in gneiss and 

 mica-slate are either distinctly separated from the bounding rock, 

 or they are intermixed with it at their line of junction ; and some- 

 times it is intermixed with hornblende, and gradually passes into 

 the hornblende rocks. 



hare tin- stratified structure, and the strata arc variously inclined, a fart which 

 shows that the slaty and stratified structures arc sometimes Independent of succes- 

 sive deposition, and that variously Inclined strata can he formed Independent of 

 any inequality of the fundamental rock, or w itliont the aid of a suutciianeous 

 pOH er. 



