GABBROS ETC. IN SCOTLAND AND IRELAND. 53 



changes ; and from such we may often learn more of the nature and 

 mode of operation of the processes by which the changes were 

 brought about, than from a study of the final products of the 

 alteration. I have already discussed in my paper on the closely 

 associated peridotites * of the district, the nature and causes of some 

 of the mineral metamorphoses to which I refer ; and the present 

 paper is to a great extent a continuation of that already published. 



II. Geographical Distribution. 



The rocks which I now propose to describe in detail cover con- 

 siderable tracts in the Inner Hebrides and the adjoining mainland 

 of Scotland, extending to the southward over the north-eastern 

 part of Ireland. Eocks with identical characters again make their 

 appearance, however, to the northward, in the Faroe Isles, and in 

 Iceland — in the last-mentioned island covering very large areas. 



The admirable descriptions of the Icelandic basic rocks which 

 have been given by many geologists, among whom may be mentioned 

 Krug von Xidda, Sartorius von "Waltershausen, Professor Zirkel, and 

 especially M. 11. Bre'on t, enable us to show the remarkable similarity 

 in characters which exists between the basalts of Iceland and the Faroe 

 Isles on the one hand, and those of Scotland and Ireland on the other. 

 Xor are the more highly crystalline forms, approximating in character 

 to our dolerites and gabbros, altogether wanting in Iceland, though 

 they are as yet imperfectly known. Such highly crj^stalline rocks occur 

 in ejected blocks, and also forming isolated peaks, as at Baula. The 

 occurrence of these peaks of crystalline basic rocks has been inter- 

 preted as necessarily implying the existence of a series of pre -Tertiary 

 igneous rocks, around which the later volcanic masses have accu- 

 mulated ; but the examples in the British Isles may well suggest 

 a doubt as to the correctness of this explanation of their mode of 

 occurrence. The tracing-out of the relations of the different igneous 

 rock-masses in Iceland is rendered difiiciilt, not only by the existence 

 of permanent snow-fields and by the inaccessibility of large portions 

 of the island, but also by the covering of volcanic products resulting 

 from recent eruptions. There has not been, as in the British Isles, 

 a long cessation of accumulation which would permit of the un- 

 covering of the deep-seated plutouic masses by denudation. My 

 comparison of the British and Icelandic types of rock has been 

 greatly facilitated by the gifts of a number of Icelandic specimens 

 from friends who have travelled in different parts of that island ; 

 among these I have especially to mention my obligations to Professor 

 John Milne, Mr. W. Watts, and Mr. J. Starkie Gardner. 



The more carefully we study the British and Icelandic types of 

 Tertiary basic rocks, the more forcibly are we struck with their 

 essential identity in character. Not only do certain peculiarities, to 

 be hereafter noticed, again and again recur in the rocks of both 

 areas, but if we compare them with rocks of the same age and of 



* Quart. Journ. Geol. Soc. toI. xli. (1885) pp. 354-418. 

 t Notes pour servir a I'etude de la Geologie de I'lslande et des lies Foeroe 

 par R. Breon (Paris, 1884). 



