in Yorkshire and Durham. 33 
pyrites may be mentioned among the minerals frequently associated 
with the dyke. It is found disseminated through the substance 
of some decomposing varieties in considerable abundance; and 
small spangles of it may occasionally be seen in the sound spe- 
cimens, especially among the larger crystals of felspar before 
mentioned. All the dark sonorous specimens act strongly on the 
magnet; but some of the light-coloured varieties, which contain 
a great excess of decomposing felspar, do not sensibly affect it. 
The dyke is generally separated by a number of natural partings 
into large blocks, which are amorphous, prismatic, or globular. 
Near the centre they are sometimes of such entire irregularity as 
to defy all description. Not unfrequently, however, in the midst 
of this confusion we may observe traces of a prismatic form; and 
where this arrangement is most complete the prisms are always 
transverse to the dyke. Good examples of this form may be 
seen in the quarry of Preston, and in other localities above 
described*. The sides of the dyke are generally occupied by 
clusters of minute horizontal prisms, which are often seen in 
great perfection even where the central mass is amorphous. In 
the great quarry of Bolam, where the trap has extended laterally 
over the horizontal beds of sandstone and coal shale, the capping 
of basaltic rock is divided into rude columns which are perpen- 
dicular to the strata on which they rest; and, therefore, nearly 
at right angles to the prismatic blocks which lie across the 
leading dyke. This arrangement is exactly similar to that which 
takes place among some masses of ancient lava near Mount 
Vesuvius tf. 
* See-Plate II. Fig. 1. 
+ Altered beds of coal in contact with trap sometimes exhibit a similar arrangement. 
Thus at Coley Hill (Geological Transactions, vol. 1V. p. 23.) a small bed of coal abuts against 
a dyke of basalt, and near this contact, the coal is deprived of its bitumen, and arranged in 
Vol. I. Part I. E beautiful 
