422 Mineralogical Observations. [Disc. 



in that of Dumfrieshire, and the Island of Arran, there are beds of 

 a compact splintery lime-stone, which occasionally contain imbedded 

 masses of lime-stone, varying in magnitude and figure ; and even 

 whole beds occur principally composed of these masses, and have 

 thus a brecciated or conglomerated aspect. That the imbedded 

 portions are not fragments, and consequently the conglomerated 

 beds not of a mechanical nature, may be inferred from this fact, 

 that an uninterrupted transition is frequently to be observed from 

 the uniform compact lime-stone to the angular concretions or frag- 

 ments, resembling what is observed in the transition from compact 

 into granular lime-stone, or from compact into granular galena. 



10. In the great beds of trap tuff, that occur in the old red sand- 

 stone and the coal formations in the middle and southern divisions 

 of Scotland, we sometimes observe globular and other shaped 

 masses of tuff, which in a superficial view have much the appear- 

 ance of fragments or rolled masses. A careful examination of them 

 shows that they are of the same general nature with the basis in 

 which they are contained; further, that they pass gradually into that 

 basis; so that it is sometimes difficult to point out the line of sepa- 

 ration between the basis and the imbedded mass. Hence we infer 

 that they are not fragments, but formed at the same time with the 

 inclosing rock. 



11. Lime-stone sometimes occurs in the form of beds in slate- 

 clay, and occasionally imbedded in it in masses, varying in size and 

 shape. The imbedded masses are sometimes so much intermixed 

 with the slate-clay that an uninterrupted transition is to be traced 

 from the one into the other ; in other instances the imbedded 

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

 junction, and have very much the appearance of fragments ; but 

 these are not fragments, as is evident from their forming one 

 extremity of a series, the opposite extremity of which is calcareous, 

 or even pure slate-clay. 



12. Sand-stone occasionally occurs in imbedded masses in slate- 

 clay, exhibiting the same geognostic relations with the lime-stone 

 and slate-clay just mentioned. Hence these masses are to be con- 

 sidered, not as fragments, but as having been formed at the same 

 time with the inclosing slate-clay. 



II. — On Cotemporaneous Masses and Feins occurring together in the 



same Mass of Rock. 



We sometimes observe within a short space, as a few hundred 

 yards, in the same tract of country, beds, imbedded masses, and 

 cotemporaneous veins, of one rock in another : thus in primitive 

 mountains we occasionally meet with cliffs of which the predomi- 

 nating rock is gneiss, subordinate to which there occur beds, im- 

 bedded masses, and cotemporaneous veins of granite ; in other 

 instances the predominating rock is granite, in which there occur 

 beds and imbedded portions of gneiss.* Appearances of this kind 



* The cotemporaneous masses of gce'.ss in granite appear occasionally to 



