THE OLDER ROCKS AT ST. DAVId's. 1 13 



diifers only from analogous dykes on the road to Forth Clais in 

 being not vertically irruptive, but inclined at the angle of the 

 divisional planes. The second band in the quarry is thicker. It 

 was found less decomposed, but with a fracture^ reminding one of 

 trap — sub-columnar distinctly, — and lying between irregular walls 

 with re-entering angles, and apparently injected therefore with 

 force. These bands were classed originally, in the paper referred 

 to, as ^'^ashy shales,'' supposed to be interbedded in ametamorphic 

 series. There are chemical doubts which would arise — besides the 

 geological ones — as to the possibility of a thin bed of basic silicates 

 a few inches thick being enclosed between two masses of acidic 

 rock with much free quartz, without an intermediate passage or 

 obliteration of the basic characters, seeing that the metamorphic 

 action would affect both. These green bands are acknowledged 

 I believe by Dr. Hicks, in a later paper now in the press, to be of 

 igneous nature, on hearing from Professor Bonney that the one at 

 Forth Lisky, which he examined microscopically, is a dolerite. 

 For my part I hold them all to have been introduced subsequently 

 to the metamorphism of the Dimetians. As to their age, they 

 may perhaps have been injected during the Febidian period, for 

 there are somewhat similar green basic rocks in that series near 

 Rhosson Rock, &c., but this is a point for further research. 



Having begun the question of intruded rocks, perhaps it will be 

 well to notice some other cases before continuing the consideration 

 of the metamorphic (Dimetian). The case of a vertical dyke has 

 been noticed by Dr. Hicks (he, cit. p. 234), and described by 

 Professor Judd {ibid. p. 235, note) as a melaphyre or altered 

 dolerite. It is a green basic rock, and its decomposition — not to 

 be wondered at seeing its great age — I believe to be due entirely 

 to infiltering waters, because we find that the narrower the dykes 

 the more decomposed they are ; e.g., there is 'close to the main 

 dyke at Forth Clais, a few yards to the W., another about six 

 inches wide, but vertical in position : evidently connected with 

 these are, on the roadside section a few yards up the road, other 



