Effects of de- 
composition, 
34 Professor Sepewick on Trap Dykes 
Traces of the globular structure are often visible, especially 
where the trap passes into an earthy state: for many of the 
larger blocks, whether prismatic or amorphous, decompose in 
concentric crusts, which easily fall off and expose the hard 
spherical nuclet. 
These balls are particularly abundant in the old quarry of 
Coatham Stob, and are associated with some blocks of a light 
grey colour, which have an earthy fracture. Both these varieties 
are interesting. Some of the balls contain a considerable quan- 
tity of olivine, which is, if I mistake not, a very rare mineral in 
all the other localities. The light-coloured blocks have a super- 
abundance of decomposing felspar, and are partially porphyritic. 
Carbonate of lime exists in the form of distinct crystals, and is 
also disseminated through the mass; and in some instances small 
spherical concretions of compact felspar are found in a congeries 
of very minute crystals, giving to such specimens the appearance 
of an amygdaloidal structure. In other cases the concretions effer- 
vesce when first plunged into acids, are opaque from the admix- 
ture of impurities, and do not possess the characters of a simple 
mineral. 
In this dyke, as in almost every similar formation, the effects 
produced by decomposition are exceedingly varied. The compo- 
nent parts, from the centre to the surface, are in some quarries 
hard and sonorous. In others, the sides are invested with a fer- 
ruginous earthy matter which only penetrates to the depth of 
a few inches, and gradually passes into a sonorous granular rock. 
Not unfrequently, a decomposing crust of more considerable 
thickness covers the surface even of the blocks which are derived 
from the center of the dyke. A number of white spots, probably 
beautiful small horizontal prisms. Under the overlying mass in the quarry of Bolam, the 
carbonaceous shale is rudely prismatic; and in one or two places where this structure is best 
exhibited, the prisms are nearly vertical. 
