Ch. XXIX.] ROCKS ALTERED BY TRAP DIKES. 013 



jacent rock or the walls of a fissure, is by no means uncommon. A 

 fine example is seen in another dike of greenstone, 10 feet wide, in 

 the northern suburbs of Christiania, in Norway, of which the forego- 

 ing figure (683) is a ground plan. The dike passes through shale, 

 known by its fossils to belong to the Silurian series. In the black 

 base of greenstone are angular and roundish pieces of gneiss, some 

 white, others of a light flesh-color ; some without lamination, like 

 granite, others with lamina?, which, by their various and often oppo- 

 site directions, show that they have been scattered at random through 

 the matrix. These imbedded pieces of gneiss measure from 1 to 

 about 8 inches in diameter. 



JRocl's 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 melted matter 

 and the entangled gases might be expected to cause. 



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

 sea, 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 perpendicularly, are altered to a dis- 

 tance of 30, or even, in some places, to 35 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 Producti, 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 con- 

 fined to those portions of the rock affected by the dike.f Some gar- 

 nets contain as much as 20 per cent, of lime, which they may have 

 derived from the decomposition of the fossil shells or Producti. The 

 same mineral has been observed, under very analogous circumstances, 

 in High Teesdale, by Professor Sedgwick, where it also occurs in 

 shale and limestone, altered by basalt.^ 



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 8 or 10 feet from the wall of the dike, being- 

 greatest near the point of contact, and thence gradually decreasing 



* Cambridge Transactions, vol. i. p. 402. 



f Ibid., vol. i. p. 410. % Ibid., vol. ii. p. 175. 



