PART I. CHAPTER VIII. 



107 



Rocks altered by Dikes. 



Deen scattered at random through the matrix. These imbedded 

 pieces of gneiss measure from one to about eight inches in 

 diameter. 



Rocks altered by volcanic dikes. — After these remarks on 

 the form and composition of dikes themselves, I shall describe 

 the alterations which they sometimes produce in the rocks in 

 contact with them. The changes are usually such as the intense 

 heat of the melted matter and the entangled gases might be 

 expected to cause. 



Plas-Newydd. — A striking example, near Plas-Newydd, in 

 Anglesea, has been described by Professor Henslow.* The dike 

 is 134 feet wide, and consists of a rock which is a compound of 

 felspar and augite (dolerite of some authors.) Strata of shale 

 and argillaceous limestone, through which it cuts perpendicu- 

 larly, 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 trap, 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 distinctly confined to those portions of the 

 rock affected by the dike.f Garnets have been observed, under 

 very analogous circumstances, in High Teesdale, by Professor 

 Sedgwick, where they occur in shale and limestone, altered ' by 

 basalt.f 



Antrim — In several parts of the county of Antrim, in the 

 north of Ireland, chalk with flints is traversed by basaltic dikes. 

 The chalk is there converted into granular marble near the 

 basalt, the change sometimes extending eight or ten feet from the 

 wall of the dike, being greatest near the point of contact, and 

 thence gradually decreasing till it becomes evanescent. " The 

 extreme effect," says Dr. Berger, " presents a dark brown crys- 

 talline limestone, the crystals running in flakes as large as those 

 of coarse primitive (metamorphic) limestone ; the next state is 

 saccharine, then fine-grained and arenaceous ; a compact variety, 



* Cambridge Transactions, vol. i. p. 410. 



t Ibid. vol. ii. p. 175- 



