M. Keilhau's Theory of Granite^ and other Rocks, 87 



series of strata, there the granite and sienite immediately dis- 

 appear, and porphyry now becomes the bounding rock ; a rule 

 from which I know only one exception, and that is at the bot- 

 tom of Sande-fiord, where granite comes in contact with quartz- 

 ose sandstone ; but this case is accompanied by particular cir- 

 cumstances, into the nature of which I cannot at present more 

 particularly enter. A similar association of transition- sand- 

 stone and porphyry has been observed out of Scandinavia ; and 

 on this point, I need only refer to the porphyries which occur 

 in such abundance in the great coal-formation. When we 

 treated of the coal -formation, we did not venture to deduce, 

 from the constant association of the massive rocks with certain 

 members of the coal group, a connection by formation between 

 the unstratified and stratified masses ; we merely remarked that 

 such an opinion had previously existed. On that opinion we 

 are now perhaps inclined to lay more weight. 



If, in our transition group, the dependence of the porphyry 

 on the sandstones has once been admitted, from analogy it can- 

 not be denied that a similar dependence exists between the gra- 

 nitic rocks and the clay-slate. But the most striking of all is 

 the connection between eurite-porphyry when it occurs in beds, 

 and alum-slate. It is possible that this last union has an en- 

 tirely different cause from that of the combination of the great 

 porphyry and the sandstones. Alum-slate, for reasons which 

 shall afterwards be explained, occurs especially near the primary 

 rocks ; hence, if the bed-like masses of eurite-porphyry, whether 

 from a general or special cause, belong particularly to the lower 

 portion of the series of transition-strata, so will they, as is actually 

 the case, be often found along with the alum-slate, without imply- 

 ing the dependence of the one formation or the other. But for the 

 moment, let that be as it may, certain it is that eurite-porphyry 

 or a similar rock as its representative, is perfectly regularly 

 found in all the places where we see alum-slate in the neigh- 

 bourhood of the primary basis ; there we find it more or less 

 perfectly in the form of beds, quite as a determinate member of 

 the formation, and alternating with the other transition strata. 

 As already stated, it always occurs in the oldest portion of the 

 transition-rocks, sometimes even directly on the outgoings of 

 the primary strata ; so that (as is shewn by later denudations) 



