480 ROCKS ALTERED BY TRAP DIKES. [Ch. XXIX 



and composition of dikes themselves, I shall describe the alterations which 

 they sometimes produce in the rocks in contact with them. The changes 

 are usuall}^ such as the intense heat of melted matter and the entangled 

 gases might be expected to cause. 



Plas-JSfeioydd. — 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 (do- 

 lerite of some authors). Strata of shale and argillaceous hmestone, 

 through which it cuts perpendicularly, are altered to a distance 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 appear- 

 ance 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 

 Some garnets 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.J; 



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 some- 

 times extending 8 or 10 feet from the wall of the dike, being greatest 

 near the point of contact, and thence gradually decreasing till it becomes 

 evanescent. " The extreme efl'ect," says Dr. Berger, " presents a dark 

 brown crystalline 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, having 

 a porcellanous aspect and a bluish-gray color, succeeds : this, towards the 

 outer edge, becomes yellowish-white, and insensibly graduates into the 

 unaltered chalk. The flints in the altered chalk usually assume a gray 

 yellowish color."§ All traces of organic remains are efl"aced in that part 

 of the limestone which is most crystalline. 



The annexed drawing (fig. 631) represents three basaltic dikes tra- 

 versing the chalk, all within the distance of 90 feet. The chalk contigu- 

 ous to the two outer dikes is converted into a finely granular marble, m m, 

 as are the whole of the masses between the outer dikes and the central 



* Cambridge Transactions, vol. i, p, 402, f Ibid. vol. i. p. 410. 



X Ibid. vol. ii. p. 175. 



§ Dr. Berger Gaol. Trans, 1st scries, vol. iii. p. 112. 



