bbb J. W. JUDD ON THE SECONDARY ROCKS OF SCOTLAND. 



several geological horizons are particularly valuable and satisfactory 

 — the more so from his well-known extensive knowledge of Jurassic 

 palaeontology. 



It may be right to mention that the discovery by myself of Creta- 

 ceous and Carboniferous strata in the Western Islands has already 

 been announced, though without any details being given — the first 

 in a letter to Sir Charles Lyell, which was read at the meeting of 

 the British Association in 1872 by Professor Hughes*, the latter in 

 a letter to the ' Geological Magazine 'f; while the general nature of 

 these recent discoveries and their bearing on the geology of the 

 Highlands have been discussed by the Duke of Argyll, in his paper 

 read at the meeting of the British Association at Glasgow in 1876$. 



III. Distribution and Physical Relations of the Secondary 

 Strata oe the Western Highlands. 



In the last published part of this memoir I endeavoured to give 

 such a sketch of the relations of the Tertiary volcanic rocks to the 

 strata of Mesozoic age as would enable the reader to realize those 

 peculiar accidents by which the preservation of the numerous isolated 

 fragments of the latter have been determined. I showed that at the 

 commencement of the Tertiary epoch considerable areas in the north 

 of Scotland must have been covered by deposits of Secondary age, 

 attaining in places to a very great thickness, and that during Eocene 

 and Miocene times there had occurred, in the district of the Western 

 Highlands, a series of volcanic outbursts on the very grandest scale ; 

 that these tremendous exhibitions of igneous violence were succeeded, 

 probably during the Pliocene period, by numerous smaller and spo- 

 radic eruptions ; and that, as the consequence of this prolonged 

 volcanic action, portions of the Secondary strata were in some cases 

 buried under enormous thicknesses of slowly accunmlated lava-sheets, 

 while other portions were caught and entangled between the intru- 

 sive sheets proceeding from the great centres of igneous activity. 

 In some instances, as I then pointed out, the Secondary rocks were 

 subjected to metamorphic action, through their contact with the 

 intrusive plutonic masses, and occasionally the metamorphism thus 

 produced was of the most extreme character : in other cases the 

 Secondary strata were broken up by the explosive action of the vol- 

 cano, so that we now find only their fragmentary relics imbedded in 

 the ejected agglomerates constituting portions of the old cones of 

 eruption ; and it is from these fragments alone that the geologist is 

 enabled to form a judgment concerning the nature and fossil con- 

 tents of the rocks through which the volcanic agencies forced vents. 

 But in every case it is in consequence of the extensive denudation 

 to which, as we have shown, the volcanic rocks of the Hebrides 

 have been subjected, during comparatively recent times, that the 



* Eep. Brit. Assoc. (1872), Transactions of Sections, p. 115. 



t Vol. i. new series, p. 573. 



X Eep. Brit. Assoc. (1876), Transactions of Sections, p. 83. 



