122 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



III. THE BOSSES AND SHEETS OF GABBRO. 



In singular contrast to the nearly flat basalts of the plateaux, another series of rocks 

 rises high and abruptly above these tablelands into groups of dome-shaped, conical, spiry, 

 and rugged hills. It is these heights which more than any other feature relieve the 

 monotony of the wide areas of almost horizontal stratification so characteristic of the 

 volcanic region of the north-west. Their geological structure and history are much less 

 obvious than those of the bedded basalts. Their mountainous forms at once suggest a 

 wholly different origin. Some portions of them have even been compared with the 

 oldest or Archaean rocks.* That they are really portions of the Tertiary volcanic series, 

 and that they reveal a wholly distinct phase in the history of volcanic action, is now 

 frankly admitted. Whether we regard them from the petrographical or structural point 

 of view, they naturally arrange themselves into two well-defined groups. Of these one 

 consists of highly basic compounds, of which olivine-gabbro is the most prominent. The 

 other comprises numerous varieties — granite, granophyre, felsite, quartz-porphyry, 

 trachyte, pitchstone, and others — all of them being decidedly acid, and some of them 

 markedly so. For reasons which will appear in the sequel, the former group must be 

 considered as the older of the two, and it will therefore be described first. 



§ 1. Petrography. 



Since the publications of Macculloch, the occurrence of beautiful varieties of highly 

 basic rocks among the igneous masses of the Western Isles has been familiar to geologists. 

 They were named by him "hypersthene rock" and "augite rock," t names which 

 continued in use until 1871, when my friend Professor Zirkel published the results of 

 his tour through the west of Scotland, and showed that the rocks in question were 

 mostly true gabbros. | Since his observations were published some of these rocks have 

 formed the subject of important papers by Professor Judd. § 



The general petrographical characters of the gabbro areas of western Scotland may be 

 summarised as follows : — A very considerable variety of petrological structure and 

 chemical composition is observable among the rocks. At the one end of the series are 

 compounds of plagioclase and augite, which, though wanting in olivine, have the general 

 structure and habit of dolerites. At the other end are mixtures wherein felspar is scarce or 

 absent, and where olivine becomes the chief constituent. Between these two extremes are 

 many intermediate grades, of which the most important are those containing the variety of 

 augite known as diallage and also olivine. These are the olivine-gabbros, which form so 



* This was my own first impression, when I began, as a boy, to ramble among them. Macculloch had correctly 

 grouped them with the other overlying rocks, and this conclusion was afterwards confirmed by Zirkel. 

 t Western Islands of Scotland, vol. i. pp. 385, 484. 

 X Zeitschrift. Deutsch. Geol. Gesellsch., xxiii. (1871) p. 1. 

 § Quart. Jour. Geol. Soc, xli. (1885) p. 354, xbi. (1886) p. 49. 



