Ch. XV.] OF THE PRIMARY ROCKS. 343 



Coll, where the central part of the nodules consists of horn- 

 blende-rock, this difference can be explained without con- 

 sidering that these nuclei have been derived from fragments 

 of the latter rock ; for the characteristic mineral of this gneiss 

 is hornblende; and that it has been attracted to certain 

 centres, and there formed by its predominance a hornblendic 

 rock, is in perfect accordance with the patches or concretions 

 which occur in granite, and all the primary rocks: and it 

 must be remembered, that insulated nodules, patches, short 

 layers or veins, and other irregular forms of the hornblende- 

 rock, are characteristic of the adjacent strata of gneiss ; the 

 conglomerated stratum only differing from their concretions, 

 by being so abundant as to prevail over the ordinary struc- 

 ture. Again, turn to the description of the conglomerated 

 mica-slate of Drontheim ; its nodules are also formed of the 

 same constituent parts as the containing rock, but they ex- 

 hibit a different texture, owing to their mode of aggregation : 

 the mica is intimately diffused throughout the schistose basis ; 

 but in the nuclei of the nodules it is in a great measure 

 wanting ; thence their compact structure, and, as if to com- 

 pensate for this deficiency in the nuclei, it predominates in 

 the micaceous laminae by which the nuclei are concentrically 

 enveloped. And the structure of the conglomerated chlorite- 

 schist of Relistian is very analogous to that of this Nor- 

 wegian rock. But it may be argued, that it frequently 

 happens that these conglomerates are formed of quartz-no- 

 dules, as is also the case with similar rocks in secondary 

 formations. It does so : but if the former be minutely ex- 

 amined, it is very probable that their nodules will exhibit 

 characters incompatible with a mechanical origin, such as 

 the penetration of them by one or other of the constituent 

 minerals of the containing rock ; as in the case of the schist 

 in the Cotentin, where talc is blended with the quartzose 

 concretions, as described by Brongniart ; who considered it a 

 proof that the nodules were formed contemporaneously with 

 the strata in which they are enveloped : or the nodules them- 



