Magnetic. 
Veins in the 
Trap. 
166 Professor’ SEDGWICK on the 
found a few concretions or irregular veins, of a much coarser 
and more decomposing variety of rock, in which the crystals of 
pyroxene are large and abundant. This mineral, in such cases, 
often puts on the form of irregular prisms, or lengthened tabular 
crystals, the planes of which are bent and undulating. As I had 
some doubts respecting the true nature of these crystals, some of 
which reach the length of two inches, I requested my friend 
Mr. W. Phillips, to undertake their examination, and he deter- 
mined, by their cleavage, and by the reflecting goniometer, that 
they are pyroxene—that they cleave easiest parallel to the 
plane P, which is uncommon, and that the broad surfaces of 
the long crystals, are not primary planes, but represent the 
plane h. (See Phillips’ Mineralogy, 3d Edit. p. 59. Fig. 2.) 
In the specimens last described, the pyroxene is of a black 
or brownish black colour, rarely of a yellowish brown. The 
surfaces are, in some cases splendid, in others they have an 
imperfect pseudo-metallic lustre or tarnish, and not unfrequently 
the crystalline faces are disguised by a coating of oxide of iron. 
Both the small-grained and the large-grained varieties of 
the trap, act vigorously on the magnetic needle, an effect pro- 
bably due to very small grains of titaniferous iron, disseminated 
through the mass. I did not, however, observe any instances 
of those large concretions of this mineral, which so constantly 
occur in rocks, composed of diallage and feldspar, belonging te 
formations of serpentine. 
The passage of a vein or faulé through the trap (as well as 
through the other strata of the country) is marked by the pre- 
sence of a number of thin vertical beds; some portions of which, 
when struck with the hammer, fall into irregular solids, termi- 
nated by plane surfaces, which are partially coated over with 
oxide of iron. But a clean fracture of most of these masses, 
exposes a very fine granular texture, and a bluish black colour, 
in which the distinction between the light and dark constituents 
