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 Newydd. 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 

 Newydd, 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 Productce, 

 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 



