358 PROF. J. W. JUDT) ON THE TEETIAET AND 



igneous group. In 1865, Dr. Haughton paid a brief visit to Sk3'e, 

 and on his return expressed the same opinion concerning the origin 

 and age of these rocks *. That the Archaean age and metamorphic 

 origin of the rocks in question is no longer maintained by the fij'st- 

 mentioned author, is evident from several of his later writings f ; 

 nevertheless, the erroneous identification has been adopted by Dr. 

 Sterry Hunt J, who makes a strong point of it as supporting his 

 views concerning the chronological classification of rocks, based on 

 their mineralogical characters. It is but just, however, to this 

 author to conclude that his acquaintance with the rocks of Skye is 

 probably limited to hand-specimens. 



The gabbros of the Inner Hebrides present, as I have already 

 shown, all the distinctive characteristics of igneous eruptive masses. 

 They are, as a matter of fact, remarkably free from all traces of the 

 foHated structure which is sometimes found superinduced in igneous 

 rocks of greater geological antiquity. In their general relations, as 

 well as in their structure, they present the most remarkable analogy 

 with the granites, of which they are clearly the representatives in 

 the basic series of rocks. Thus they form mountainous masses inter- 

 sected in all directions by the so-called " contemporaneous " or 

 " segregation " veins, and are sometimes studded with enclosures 

 composed of the same minerals as the rock itseK, but in diff'erent 

 proportions. These gabbro-masses become finer-grained and less 

 perfectly crystalline towards their edges, and they give off innumer- 

 able veins into the surrounding rocks, while the most striking eifects 

 of contact-metamorphism are seen in stratified materials lying in 

 juxtaposition with them. 



The dolerites of the Western Isles of Scotland form smaller erup- 

 tive masses, and also great intrusive sheets which are not only seen 

 to be connected with the larger gabbro-intrusions, but graduate into 

 them in the most insensible manner. Some of the thickest of the 

 lava-flows are also dolerites, undistiuguishable in their characters 

 from the intrusive masses. 



Locally, both gabbros and dolerites are found passing into perido- 

 tites by the gradual disappearance of the felspar and the increase in 

 quantity of the olivine. In this way the felspathic rocks are seen 

 graduating insensibly into the non-felspathic forms, and these latter, 

 in the island of Eum, cover vast areas and form the bulk of moun- 

 tain-masses thousands of Yeet in height. It is noteworthy that the 

 peridotites make their appearance not only in the deepest valleys of 

 E,um, but also on the top of Halival and other mountains in the 

 island which are between 2000 and 3000 feet high. 



But there is another and very interesting mode of association of 

 the peridotites with the gabbros and dolerites, which is especially 

 worthy of notice. The peridotites are often found in segregation- 

 nodules in the gabbros, and occasionally segregation-nodules of 

 gabbro are found in the peridotites, the phenomena being strikingly 



* Dublin Quart. Journ. Sci. vol. v. (1865), p. 94. 



t A. Geikie, ' Text-Book of Geology ' (1882), p. 150. 



+ Sterry Hunt, ' Chemical and Geological Essays,' 2ncl edition (1879), p. 279. 



