252 Prof. Sedgwick on some Trap Dykes 
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 nuclei. 
These balls are particularly abundant in the old quarry of 
Coatham Stob, and are associated with some blocks of a light 
gray colour, which have an earthy fracture. Both these va- 
rieties are interesting. Some of the balls contain a consider- 
able quantity of olivine, which is, if I mistake not, a very rare 
mineral in all the other localities. The light-coloured blocks 
have a superabundance of decomposing felspar, and are par- 
tially 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 struc- 
ture. In other cases the concretions effervesce when first 
plunged into acids, are opaque from the admixture of impuri- 
ties, and do not possess the characters of a simple mineral. 
Effects of Decomposition. 
In this dyke, as in almost every similar formation, the effects 
produced by decomposition are exceedingly varied. The com- 
ponent parts, from the centre to the surface, are in some 
quarries hard and sonorous. In others, the sides are invested 
with a ferruginous 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 centre of the dyke. A 
number of white spots, probably resulting from decomposing 
felspar, are often disseminated through these earthy masses, 
and enable us to separate them from other argillaceous ma- 
terials, with which they are sometimes in contact. It would 
be a laborious, and not a very profitabie task, to attempt a 
minute account of phenomena like these which vary with every 
different locality, 
Liffects produced by Contact of the Dyke. 
It now remains to describe some of the effects produced by 
the intrusion of the dyke. These effects will of course vary 
with the substances which are acted on. In some of the quarries 
the coal is deprived of its bitumen, and arranged in beautiful small hori- 
zontal prisms. Under the overlying mass in the quarry of Bolam, the car- 
bonaceous shale is rudely prismatic: and in one or two places where this 
structure is best exhibited, the prisms are nearly vertical. 
which 
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= Se oe 
