( 359 ) 



XXI. — The Old Red Sandstone Volcanic Rocks of Shetland. By B. N. Peach 

 and J. Hokne, of the Geological Survey of Scotland. (Plates XLV. and 

 XLVI.) 



CONTENTS, 



PAGE 



Introduction, 359 



I. Geological Structure op the Volcanic 



Rocks, 361 



A. Contemporaneous Lavas and Tuffs, . 361 



1. Porphyritic Lavas and Tuffs of North- 



mavine, . . . .361 



2. Diabase Lavas and Tuffs of Aithsting 



and Sandsting, .... 364 



3. Probable Horizon of the Lavas and 



Tuffs of Aithsting and Sandsting, 365 



4. Lava of the Holm of Melby, . . 366 



5. Bedded Lavas and Tuffs of Papa 



Stour, 366 



PAGB 



5. Necks on Bressay and Noss, . . 377 



6. Summary of Events indicated by 



Volcanic Phenomena of Shetland 



Old Red Sandstone, . . . 378 



6. Band of Tuff in Bressay, 

 B. Intrusive Igneous Rocks, .... 



1. Intrusive Sheet of Binary Granite 



in Northmavine, .... 



2. Intrusive Sheet of Granite in Sand- 



sting, 370 



3. Intrusive Sheet of Spherulitic Fel- 



site in Papa Stour, 



4. Dykes — .... 



a. Binary Granites, 



b. Quartz-felsites, 



c. Rhyolites, 



d. Diabase Rocks, 



367 

 368 



368 



371 

 373 

 373 

 373 

 374 

 376 



II. Microscopic Characters. 



1. Porphyrite Lavas, 



2. Diabase Lavas, 



3. Intrusive Diabase 



Rocks, 



4. Intrusive Sheets — 



Rooeness Sheet, 

 Sandsting Sheet, 

 Papa Stour Sheet, 



5. Dykes — 



Binary Granites 

 and Quartz-fel- 

 sites, 



Rhyolites, 



Basic Rocks, 



Acidic Rocks, 



379 

 379 



381 

 382 

 382 

 382 

 383 

 383 



383 



384 



Summary of Results, 386 



Appendix. — Table of Chemical Analyses of 

 eight Specimens of Shetland Old Red 

 Volcanic Rocks, by R. R. Tatlock, 

 F.R.S.E., 387 



Perhaps the most interesting feature connected with the Old Red Sandstone 

 formation in Shetland is the evidence of prolonged volcanic activity in those 

 northern isles. The great development of contemporaneous and intrusive 

 igneous rocks, which gives rise to some of the most striking scenery in Shet- 

 land, is all the more important when compared with the meagre records in the 

 Lower Old Red Sandstone of Orkney and the Moray Firth basin. Not till we 

 pass to the south of the Grampians do we find evidence of a far grander 

 display of volcanic action during this period, in the sheets of lava and tuff in 

 the Sidlaws and Ochils and in the great belt stretching from the Pentlands 

 south-westwards into Ayrshire. The relations of the Shetland igneous rocks 

 are admirably displayed in the various coast sections, especially in the mural 

 cliffs of Northmavine and some of the Western Islands. From these records, 

 though they have been subjected to much denudation, it is possible to con- 

 struct a tolerably complete sketch of the volcanic history of this formation, as 

 developed in that region. 



VOL. XXXII. PART II. 3 M 



