50 GEOLOGY OF THE LOTHIANS. 



associated with radiated haematite. Quartz occurs in drusy 

 cavities, being either common or amethystine, and is crystal- 

 lized in the usual six-sided pyramid. Prehnite, of various 

 shades of yellow and green, occurs in the same position, and 

 is arranged in botryoidal or imperfectly crystallized masses. 

 Cubicite or Analcime, is also generally associated with it. A 

 datholitic mineral (as mentioned by Professor Jameson), 

 to which the name of Humboldite has been given, was 

 also at one time found in the same situation. Iron-py- 

 rites is very generally distributed through the greenstone, 

 and on decomposition causes the rock to assume a brown- 

 ish-yellow colour. Sulphate of barytes and agaric mine- 

 ral occur associated with the sandstone ; the former con- 

 stituting minute veins, while the latter encrusts its exposed 

 surface. 



Separated from Salisbury Crags, by the valley of the 

 Hunters' Bog, the hill of Arthur's Seat rises.* Taken as 

 a whole, it appears to rest upon Salisbury Crags, and is, 

 therefore, consequently of newer formation. The various 

 details connected with it are perhaps not so satisfactory as 

 those of the former ; they are, however, in many respects 

 interesting. At the eastern extremity of the Hunter's Bog, 

 sandstone occurs lying under a mass of porphyritic green- 

 stone, and both dip to the S. E. at 20°. This sandstone may 

 be traced, for a considerable way, lying under the porphyry, 

 in which numerous masses of the sandstone occur imbedded. 

 One of these masses is seventy feet long by two in breadth ; 

 several of a smaller size also occur completely enveloped in 

 the trap, one of which is visible for about twenty-five 

 feet. (Plate VI. Fig. 2.) When in contact with the ig- 



* The first published account of the geognosy of this hill and of Salis- 

 bury Cragsappeared in Professor Jameson's Mineralogy of Dumfriesshire ; 

 the description here given is nearly that which the Professor now deli- 

 vers to his pupils during his geological excursions. 



