146 
Professor Jameson q?i 7\ajp-Veifis. 
numerous instances of veins, both metalliferous and saxigenous^^ 
in which the crop or outgoing is wide, but the lower part nar- 
row, and terminating in a wedge, or in numerous small branches. 
Both statements are correct, and, therefore, the two opinions 
appear to be plausible. But there is a fact which cannot be re- 
conciled with either of the hypotheses, and which forces us to 
have recourse to some other mode of explaining the formation 
of these veins. The fact we allude to is the occurrence of veins 
that terminate both above and below in the rock which they tra- 
verse, in short, are completely inclosed in it. Fig- 7. Plate III^, 
represents the three kinds of veins; a, vein open at top ; 6, vein 
shut above, and c, vein terminations in the rock. Such a vein, 
it is evident, could not have been filled from above, nor is it possi- 
ble to conceive that it could have been filed from below. We 
are therefore led to the conjecture, that such veins must have 
heen formed at the smne time with the roch in which they are 
contained.’’'' This view receives considerable support by a care- 
ful attention to the appearances presented by the distinct con- 
cretions into which many trap-rocks are naturally divided. 
These concretions, at their line of junction with the bounding 
trap-rock, exhibit the same phenomena as occur at the meeting 
of the sides and walls of trap-veins in trap r and they differ 
from veins only in shape ; the concretions being short and 
massive, whereas the veins are long and tabular, A vein of 
trap (or a dike of trap) we consider as merely a series of con- 
cretions, arranged in a tabular form. But, it will be inquired 
how, on this principle, we can explain the crossing of trap-veins 
This arrangement we consider as an effect of crystallization, and 
of precisely the same nature as the crossings observed in crystals, 
or the intersections in tabular concretions. The crossings in 
crystals require no illustration,, but those of concretions may be 
shortly described. In sections of trap-rocks, as of greenstone^ 
sometimes the cliff is naturally divided, into tabular concretions, that 
extend from the top to the bottom of it. Tliese concretions occa- 
sionally vary in their direction, some being horizontal, and others 
vertical ; but the remarkable fact, and that which is highly illus- 
trative of our present view, is this, — that the same table or tabu- 
lar concretion will continue for some feet or yards parallel with 
the bounding concretions, and then suddenly turn and intersect 
these for several yards or fathoms, and then terminate, as 
a a. Fig. 8, Plate III. Now, this concretion in one part of 
