368 
ALTERATIONS PRODUCED 
[Ch. XXVI. 
theory by direct experiments on the fusion of rocks in the labo- 
ratory, and by observation of the changes in the composition 
and texture of stratified masses, as they approach or come in 
contact with igneous veins and dikes. In studying the latter 
class of phenomena, we have the advantage of examining the 
condition of the rock at some distance from the dike where it 
has escaped the influence of heat, and its state where it has 
been near to, or in contact with, the fused mass. The changes 
thus exhibited may be regarded as the results of a series of 
experiments, made on a great scale by nature under every 
variety of condition, both as relates to the mineral ingredients 
of the rocks, the intensity of heat or pressure, the celerity or 
slowness of the cooling process, and other circumstances. 
Strata altered by volcanic dikes — Plas Neivydd. — We shall 
select a few examples of these alterations in illustration of our 
present argument. One of the most interesting is the modifi- 
cation of strata in the proximity of a volcanic dike near Plas 
New}fdd, in Anglesea, described by Professor Henslow. The 
dike is 134 feet wide, and consists of basalt (dolerite of 
some authors), a compound of felspar and augite. Strata of 
shale and argillaceous limestone, through which it cuts perpen- 
dicularly, are altered to a distance of thirty, or even in some 
places to thirty-five feet, from the edge of the dike. The 
shale, as it approaches the basalt, becomes gradually more 
compact, and is most indurated where nearest the junction. 
Here it loses part of its schistose structure, but the separation 
into parallel layers is still discernible. In several places the 
shale is converted into hard porcellanous jasper. In the most 
hardened part of the mass the fossil shells, principally Products, 
are nearly obliterated, yet even here their impressions may 
frequently be traced. The argillaceous limestone undergoes 
analogous mutations, losing its earthy texture as it approaches 
the dike, and becoming granular and crystalline. But the 
most extraordinary phenomenon is the appearance in the shale 
of numerous crystals of analcime and garnet, which are 
