ISLANDS IN THE FIRTH OF FORTH. 75 



with the sandstone strata ; and in the bed of the Gogar 

 burn at Wester Haggs, another dyke traverses the sand- 

 stone vertically, which abutts against it on both sides. At 

 Stoneyrig also, a similar dyke occurs; there are no junc- 

 tions visible, but the strata are fractured, contorted, and 

 inclined at high angles. 



Having now described the relations of the trap-rocks which 

 occur in the more immediate vicinity of Edinburgh to the 

 various Neptunian strata, the present appears a convenient 

 place to introduce a description of the islands which rise 

 above the waters of the Firth of Forth, in as much as 

 there are circumstances which connect them with the seve- 

 ral rocky masses which we have just noticed. Of these 

 islands, two only, Inchkeith and Inchcolm, contain strati- 

 fied rocks, and both exhibit several interesting appear- 

 ances. The strata which occur in Inchkeith consist of 

 limestone, calcareous sandstone, slate-clay, and bitumi- 

 nous shale, all of which dip to the S. S.W.* "fThe ig- 

 neous rocks of this island are amygdaloid, earthy green- 

 stone, basalt, and tufa, and at the lines of junction with 

 the stratified rocks there are produced all those changes 

 which so frequently occur ; the slate becomes indurated, 

 the limestone crystalline, and the sandstone quartzose. In 

 several places, masses of the stratified rocks are carried up, 

 and completely enveloped in the trap ; these masses varying 

 from the smallest size to the magnitude of many yards. On 

 the eastern side of the island the columnar mode of arrange- 

 ment is well displayed in the greenstone. The tufa which 

 occurs in Inchkeith is formed by a base of earthy trap con- 

 taining variously sized masses of basalt, greenstone, sand- 



* Vide pp. 216, 17, 18, 19, and 220 of Dr Neill's translation of Dau- 

 buison's Memoir on Basalt, for an interesting account of the geognosy of 

 the coast of Fyfe. 



