88 M. Keilhau's Theory of Granite, and other Rocks. 



it is only on the hanging side that it is connected with the Nep- 

 tunian transition-strata. In regard to this subject, we have to 

 mention a very remarkable circumstance, — that the petrographi- 

 cal characters of the rocks are modified in a determinate man- 

 ner, according as the masses lie quite near the basis of the 

 transition formation, or are found somewhat higher up in the 

 series of beds. In the first case, it presents itself as almost a 

 pure quartz, as an impure ochre-coloured rock, in which fel- 

 spathic substances are very rarely present, and in which felspar 

 crystals are often entirely awanting : but higher up in the range 

 of beds, the basis or ground of the rock acquires more and more 

 felspathic matter, and first of all becomes euritic, and afterwards 

 quite crystalline. At a still greater distance from the primary 

 series, the same rock becomes of a sienitic character, but then 

 it can no longer be said to have a fixed place in the succession 

 of strata. The regularity of these progressive changes is limited 

 only by another likewise very remarkable rule, that the larger 

 masses are more crystalline, and richer in felspar, as well in 

 homogeneous distribution in the basis or ground, as in ihe crys- 

 tals, than the smaller ones ; so that a perfect porphyry may 

 present itself very near the primary rock, provided the mass 

 be large enough, while a compact and rather quartzose than 

 euritic rock may occur further on in the range of beds, because 

 it exists in a comparatively thin bed. That the larger masses 

 are more crystalline than the smaller^ accords perfectly well 

 with Vulcanism ; but certainly the other relations we have men- 

 tioned, cannot thus be made to correspond, for it seems incon- 

 trovertible that this remarkable family of the massive rocks, — 

 whether in the form of quartz, or of sienite, or of true eurite 

 porphyry, cannot have been intruded from without into our 

 formation, since they could as well have appeared elsewhere, 

 «ven beyond the limits of the formation ; and it is also clear 

 that they are associated in a peculiar manner, which banishes 

 the idea that the masses belonging to them have taken the places 

 where they are found accidentally. 



These, now, are facts of a new kind,' and it is a fortunate 

 circumstance that they can be observed in our own immediate 

 neighbourhood, although the full conviction of their accuracy 

 can only be derived from the examination of a great number of 



