OLD RED SANDSTONE VOLCANIC ROCKS OF SHETLAND. 



377 



felsite through which these diabase rocks are injected are occasionally very 

 hornblendic, but not always so. Notwithstanding this feature, the boundary is 

 clearly defined. On the Skeld Hill the boss of diabase has a well-marked 

 boundary separating it from the granite which surrounds it. In this instance 

 the granite is micaceous, with pink orthoclase felspar and quartz, the mica 

 being very dark-coloured and abundant close to the basic rock. The diabase, 

 on the other hand, hardly contains any quartz, and the felspar is only 

 sparingly developed ; while the green mineral is very abundant. In a subse - 

 quent paragraph we shall point out the resemblance in chemical composition, 

 and, to a certain extent, in microscopic characters, between these basic 

 intrusions and some of the diabase lavas. From the manner in which they 

 traverse the intrusive sheets, there can be no doubt that they mark a later 

 phase of volcanic activity, if, indeed, they do not mark the close of volcanic 

 action in the Old Red Sandstone of Shetland. 



3. Necks. — The occurrence of volcanic pipes of Old Red age in Shetland is 

 another proof of the manifestation of volcanic activity which characterised that 



too \ Inclined strata. 

 - J H Highly inclined do. 

 «* Vertical strata. 



C o n c 



m 



Sandstone 

 and shale. 



Porphyrite 

 dyke. 



Pig. 14. — Plan of Volcanic Pipes or Necks on East Shore of Bressay. 



period. Strange to say, they occur on the eastern seaboard of Shetland, where 

 the interbedcled volcanic materials hardly exist. So far as our observations 

 have gone, no trace of these volcanic orifices is to be found in the western 

 districts of the Mainland or the adjoining islands, where the igneous rocks are 

 best developed. At the entrance to Noss Sound, on the south-east shore of 

 . Bressay, and also in the island of Noss, a series of necks is exposed. The vent 

 in the island of Noss was noted by Dr Heddle in 1848, # though, so far as we 

 are aware, no previous description of it has been given. The necks are arranged 

 in a linear manner, and have evidently come to the surface along a line of fissure. 



* Mineralog. Mag., vol. ii. p. 253, 



rites 



