HISTORY OF ARTHUR'S SEAT. 



305 



trees in the volcanic ashes on two or three distinct horizons. The 

 height of the trunks is limited by the thickness of the ash, which is 

 three feet. 1 



Quartz-Felsite of Corriegills. Professor Bonney has drawn atten- 

 tion to the great quartz-felsite dyke in Arran, on the Corriegills shore, 

 south of Erodick, as having the base traversed by parallel joints which 

 divide the rock into plates like tiles. Higher up there is rude vertical 

 prismatic jointing, and higher still a recurrence of the platy structure. 

 Other examples of predominant fissile structure are seen in the pitch- 

 stone veins at Corriegills and Dunfion, while the latter rock at Tormore 

 is sometimes also rudely columnar. 



The great pitchstone at Corriegills shows under the microscope 

 quantities of microlithic dust, with larger belonites, either singly or 

 in groups, aggregated in patterns like algae. Sometimes the pitchstone 

 shows a rough perlitic structure. 



Fig. 59. Drumadoon (Arran). 



On the shore north of Drumadoon the felsite is divided by a dyke 

 of basalt. On the one side of the dyke the felsite is compact and 

 flaggy, but on the other side it is porphyritic. 2 



Arthur's Seat. One of the most interesting remains of an extinct 

 volcano of the latter part of the Primary period is seen in Arthur's Seat. 

 There the strata consist of sandstones and shales of estuarine origin, 

 \vhich belong to the lower part of the calciferous sandstone, and 

 alternate with stratified tuffs and sheets of dolerite and felspathic 

 lava. These rocks dip N.E. about 20, and form part of a great 

 anticlinal fold. Through these carboniferous rocks rise masses of 

 lava, dykes, and piles of volcanic agglomerates. Professor Geikie 

 formerly referred this latter outburst to the Permian period, but Pro- 

 fessor Judd has adopted the view that the interstratified lavas, and 



K l E. A. Wiinsch : Trans. Geol. Soc., Glasgow, 1865-66, p. 97. 

 - Bonney : "Pitchstones and Felsites in Arran," Geol. Mag., 1877. 

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