3 o6 THE LAVAS ROUND ARTHUR'S SEAT. 



those which formed the core of the old volcano, are portions of one 

 continued eruption ; so that, according to the latter writer, the history 

 of Arthur's Seat, after the formation of the fundamental rocks in the 

 Lower Carboniferious period, included the eruption of ashes, basalts, 

 and augite-andesites, and the injection of the great masses now known 

 as St. Leonard's Crags, Salisbury Crags, and Samson's Eibs. Fossils 

 are found in the stratified tuffs of St. Anthony's chapel, showing that 

 the eruption was at first submarine. Coarse agglomerates lie around 

 the central basaltic throat of Arthur's Seat, just as they would do 

 around the eruptive throat of a volcano, though Professor Judd sug- 

 gests that the position of the eruptive outlet may have been changed 

 from time to time. 1 



Mineral Character of the Lavas near Edinburgh. Arthur's 

 Seat exhibits examples of contemporary interbedded dolerites in the 

 masses known as Long Eow, the Haggis Knowe, and St. Anthony's 

 Chapel. The rock sometimes shows a fluidal structure, the long axes 

 of the felspar prisms being more or less parallel to each other. The 

 intrusive dolerites of Arthur's Seat form the three ridges known as 

 Mount Heriot, Salisbury Crags, and the Dassies. In the Salisbury 

 Crag bed, the felspar and augite crystals are sometimes visible to the 

 eye, and are associated with grains of calcite and analcime, with 

 prehnite and pectolite. It always contains crystals of orthoclase as 

 well as plagioclase. Calcite appears to have replaced part of the 

 glassy matrix. The Dassies consists of a rock which is partly decom- 

 posed, and is green from the chlorite developed in it. 



At Dalmahoy, eight miles S.W. of Edinburgh, the black dolerite 

 is semi-crystalline. At Ratho the rock is coarse-grained, with the 

 augite sometimes altered into a fibrous, brownish -green substance. 

 Similar rocks are seen at Springbeth, near Queensferry on the Forth. 

 At Corstorphine Hill the rock is described by Mr. Allport as a true 

 gabbro ; it is a granular compound of plagioclase, diallage, and a little 

 magnetite, with serpentine occupying the interspaces between the 

 constituents. 2 



Salisbury Crag. In Salisbury Crag is a very fine section of basalt 

 in places 80 feet thick, enclosed between stratified sandstone, con- 

 glomerate, shale, and ironstone nodules, and it is easily seen that both 

 the igneous and sedimentary rocks were altered at their formation. 

 Masses of sandstone and conglomerate, of various forms and magni- 

 tudes, are insulated in a confused manner within the basalt, and por- 

 tions of basalt interposed among the sandstones. No dyke appears ; 

 but small veins of calcareous spar, occasionally metalliferous, cross the 

 line of junction. The accompanying drawings and references will 

 sufficiently explain the most interesting phenomena observed, and 

 give a general view of the face of the cliff as it appeared to Pro- 

 fessor Phillips in 1826. The letters of reference, a, b, c, mark points 

 of which details are given below. On a nearer examination, the 

 point a shows basalt gradually changing to a red colour and finer 



1 Judd : Q. J. G. S., vol. xxxi. p. 131. 



2 Allport : Carboniferous Dolerites, Q J. G. S., vol. xxx. 





