4 GEOLOGICAL MEMOIRS. 
strasse, described hy Gustav Leonhard*, in parts of which the layers 
of the stone are not thicker than card+;—and of the slaty and 
foliated porphyry im the neighbourhood of the Lenne valley, of which 
we have an account in an imteresting memoir by Von Dechen{, but 
respecting which it may certainly be said that its nature is as yet 
very obscure. In general, porphyries more especially supply exam- 
ples of the kind of structure now under consideration ; and although 
in many instances they may be satisfactorily shown to be metamor- 
phic slates, it is no less true that in many others it is unnecessary to 
have recourse to that theory to account for their structure. 
It may therefore be considered as proved, that many rocks, which 
are undoubtedly of plutonic origin, have a most distmet parallelism 
of structure; and consequently, that the existence of such a structure 
is in all cases by no means a proof that the rock had origmally been 
sedimentary. This truth has a very important bearing upon those 
rocks, which, from their mineral composition and their frequently 
passing by insensible degrees into other rocks of acknowledged plutonic 
nature, give rise to doubts whether they have had a sedimentary 
origin or not. Gneiss, gneiss-granite, and granulite belong especially 
to this class. 
It is unquestionably true, that clay-slate and mica-slate, when in 
the vicinity of large granitic beds, very frequently exhibit a more or 
less remarkable gneiss-like structure; and also, that genuine meta- 
morphic gneiss is met with; but in all such cases the gneiss is very 
subordinate in importance ; and generally the metamorphic ehange is 
to be recognized in a very distinct, indubitable manner by a gradual 
passage from the original rock into the altered structure: moreover, 
the existence of large masses of plutonic rocks in the immediate vici- 
nity indicates the cause of the metamorphism. 
On the other hand, under what totally different cireumstances do 
those colossal beds of gneiss-hke rocks present themselves which are 
spread over vast regions; such, for example, as those in Saxony, 
Scandinavia, Finland, North America and Brazil! how different, too, 
the gneiss-granite of the Alps and the Riesengebirge! and how little 
are we justified in setting all these down as metamorphic sedimen- 
tary deposits! = . 
When we reflect on the numerous instances recorded by the most 
trustworthy observers of the above-mentioned transition of some 
granites into gneiss, on those oscillations in texture which are not 
unfrequently seen many times repeated in the same bed, one can 
scarcely come to any other conclusion than this, that gneiss and 
* Beitrage zur Geologie der Umgegend von Heidelberg, 1844, s. 29. 
+ This appearance of the porphyry of the Wagenberg was first observed by me, 
and was described in 1827 in Moné’s Badischem Archive, ii., was the subject of a 
communication to the Meeting of Naturalists in Heidelberg in 1829, and was after- 
wards noticed in 1830 in my ‘Gea Heidelbergensis,’ s.75. The rock being in 
globular concretions and the layers concentric, it could not be ascribed to a sedi- 
mentary mode of formation.—WNofe by Professor Bronn, one of the editors of the 
Jahrbuch. 
+ Karsten und v. Dechen’s Archiv, Bd. 19. s. 367. 
