in Yorkshire and Durham. 251 
mistake not, pyroxéne and felspar, in which respect it agrees 
with the greater number of trap dykes which have been care- 
fully examined, as well as with a great many varieties of re- 
cent lava. The principal modifications, of course, arise from 
the variable proportions of these essential ingredients. Among 
the prevailing and nearly compact portions of the dyke, there 
are some larger crystals of felspar and carbonate of lime; very 
rarely, however, in such abundance or order of arrangement, 
as to give any decided appearance of porphyritic structure. 
Good specimens of amygdaloid are not common; where they 
do occur, the nodules are chiefly composed of carbonate of 
lime. In one or two instances we found chalcedony filling the 
hollows of an imperfect amygdaloid. Iron 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 specimens, 
especially among the larger crystals of felspar before men- 
tioned. All the dark sonorous specimens act strongly on the 
magnet; but some of the light-coloured varieties, which con- 
tain 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 un frequently, how- 
ever, in the midst of this confusion we may observe traces of 
a prismatic form; and where this arrangement is most com- 
plete the A as are always transverse to the dyke. Good ex- 
amples 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 cen- 
tral 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 di- 
vided into rude columns which are perpendicular 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*. 
Traces of the globular structure are often visible, especially 
where 
* Altered beds of coal in contact with trap sometimes exhibit a similar 
arrangement. Thus at Coley Hill (Geological Transactions, vol. iv. p. 23), 
a small bed of coal abuts against a dyke of basalt, and near this contact 
212 the 
