regarding the Rocks of the Red Sandstone Formation. 73 
of aqueous deposites, a point which I have sufficiently elucidated 
in my Essay on Scotland. 
On the other side, again, the best proof that M. Beudant 
1 imagines he gives of the impossibility of the igneous origin of 
porphyries, is, that they are mineralogically similar to the 
porphyries, which form beds in gneiss and mica-slate,” (See Voy- 
age^ &c. vol. iii. p. 200). This comparison is, in the first place, 
contrary to sound reasoning ; and, in the second, erroneous and 
fallacious ; — contrary to sound reasoning, because it is compa- 
ring a problematical product with one of which the aqueous ori- 
gin is purely a hypothetical assumption ; for, if the gneisses and 
mica-slates were even indisputably aqueous products, which is 
by no means proved, it would not necessarily follow that the 
porphyries should have the same origin, when it is acknowledged 
that basaltic masses alternate with beds of tertiary coarse ma- 
rine limestone, or fresh water deposites. If M. Beudant deny 
this fact, or if he will not believe the geologists who have seen 
these alternations, nothing will be able to make him change his 
opinion but the inspection of the contested localities. But, be- 
sides, the comparison is quite fallacious, because those pretended 
porphyry beds are only a sort of more or less singular veins ; they 
are masses connected with extensive old porphyritic deposites, 
as I have satisfactorily shewn in the Erzgebirge, (See my Me- 
moir on Germany in the Journal de Physique^ 1822) ; and hence 
the pretended primitive porphyries are similar to those of the 
red and coal sandstone. These veins show the geognostic posi- 
tion and singular appearances of basaltic dikes, and easily de- 
ceive, when they are included in mica-slate or gneiss nearly pa- 
rallel to the plane of stratification of the beds ; but if it were 
possible to follow them over a great space, one would soon be- 
come aware of their true nature. 
I may add, also, that I deny most positively the true passage 
from a porphyry into a sandstone^ to which point M. Beudant 
could have given more importance than he has done. This 
transition (referred to at p. 193), is only apparent or quite ac- 
cidental : it is a sandstone united by means of a feldspathic 
breccia with a porphyry, or a sandstone intimately connected 
with a feldspathic agglomerate, which, by a very compact re-ag- 
gregation, presents in certain parts the false appearance of a por- 
phyry, as is often to be seen in Irachytic agglomerates. 
