older Deposits of the North of Germany and Belgium. 285 



on a physical map, is not defined by the strike of the mineral masses composing 

 it, but by the general direction of the unconformable secondary strata which sur- 

 round it. We have no example of such a chain in England. 



In mineral structure the slate rocks of the Hartz agree generally with those of 

 the Rhenish provinces. They agree also in their modifications of structure, and 

 in their modes of association with trappean rocks : and in the contortions of their 

 beds, and in the total inversion of enormous masses of them, they present the 

 same difficulties in determining the true order of superposition from the mere evi- 

 dence of vertical sections. But it is not merely from vast contortions and inver- 

 sions, and the frequent intrusion of trappean rocks (like those described in the 

 former pages of this paper), that the order of the formations is obscured. The 

 great bosses of granite also play a most important part, both in modifying the 

 structure of all the neighbouring rocks, and in producing a distinct order of dislo- 

 cations. By the combination of all these disturbing causes, the chain of the 

 Hartz has literally been broken into fragments, in such a manner that it would 

 be almost impossible to give them any symmetrical arrangement out of which we 

 might eliminate their true relations to one another*. 



Lastly, in the Hartz the older strata are on every side cut off abruptly by the 

 unconformable abutment of the secondary strata ; so that we look in vain for any 

 frontier, like that of Westphalia ; where, after escaping from a sea of troubles, 

 every formation rests in its proper order. 



Before we go on to consider the evidence offered for the age of the formations 

 of the Hartz, from analogies of structure and from fossils, we may briefly notice 

 the principal igneous rocks which enter prominently into its structure. 

 They may be divided into four distinct classes. 



1. Trappean rocks that rise irregularly among the beds, and strike nearly with them. — Some masses 

 appear to have been protruded through the aqueous deposits : others are in more regularly stratified 

 bands, passing sometimes into the state of schaalstein, and therefore apparently of a date contempo- 

 raneous with the slate rocks. From what we saw of these rocks, and still more from the specimens 

 kindly exhibited to us by Professor Hausmann, they may be described in general terms by the name of 

 greenstone, with many modifications. They are often ferruginous, and associated with deposits of iron 

 ore near their junction with the regular strata ; often very felspathic, containing here and there much 

 Saussurite and small crystals of hypersthene, and rarely schiller-spar. 



2. Granite of the Brocken, the lower part of the valley of the Bode, ^c. — That this granite is newer than 

 the rocks of the preceding class, is proved by the fact, that it cuts them all off" and shoots its veins into them, 

 or into the series of the rocks, of which they seem to form an integral partf. Neither can it admit of any 



* This opinion has been expressed by Professor Hausmann and other German authors. 



+ By a precisely similar argument, we infer that the granite of Dartmoor is newer than the trappean 

 rocks associated with the surrounding slates. On the contrary, in the Isle of Arran, the trap rocks 

 are proved to be the newest, because they cut both through the secondary rocks and the granite. 



VOL. VI, SECOND SERIES. 2 P 



