360 PKOE. J. W. JUDD ON THE TEKTIAEY AlH) 



the pyroxenes and olivines of the felspathic and the non-felspathic 

 rocks exhibit the most striking resemblances. 



To the similar relations between the altered peridotites (or ser- 

 pentines) and gabbros, which have been so frequently remarked 

 upon by Prof. Bonney * and others, I need do no more than make 

 a passing reference. 



The constant association of serpentines, which are altered perido- 

 tites, with gabbros appears to have led to the supposition, a highly 

 improbable one, that the latter rocks can be changed into serpentine. 

 The fact that the felspathic rocks graduate so constantly into the 

 non-felspathic will fully account for the association of the hydrated 

 representatives of the latter class with the more or less altered 

 representatives of the former without the necessity of our having 

 recourse to the violent hypothesis referred to. 



§ 2. Microscopic Strtjcttiee of the Kocks. 



The larger intrusive masses of basic composition in the Inner 

 Hebrides are all perfectly Tiolocrystalline, and show no vestige of a 

 glassy basis or ground-mass. Nevertheless some very interesting and 

 distinctive structures are seen to be exhibited by them, when they 

 are studied in thin sections, under the microscope. 



The structure usually ■ displayed by the gabbros maybe defined 

 as granitic, that is to saj', they are built up of crystals, the deve- 

 lopment of the outward forms of which has been to a great extent 

 prevented by the growth in juxtaposition with them of other crystals. 

 We may perhaps conclude that in these rocks the separation in the 

 magma of the several minerals, felspar, augite, and olivine, took 

 place almost simultaneously. 



Occasionally, however, the minerals of the gabbros have a ten- 

 dency to the formation of more or less rounded grains, and an ap- 

 proach to the grmiulitic structure is exhibited. 



The gabbros vary greatly in the coarseness of their grain. In 

 Skye and Ardnamurchan, we find rocks made up of crystals an inch 

 or more in length, and every gradation is seen from these coarse- 

 grained rocks down to those in which the crystals are extremely 

 minute. 



The dolerites, which are, as a general rule, finer-grained rocks, 

 exhibit two very interesting types of struct are, which seem to be 

 worthy of careful study. 



For the first of these I propose, following M. Michel Levy, to 

 employ the term opliitic structure, from its having been first noticed 

 in connexion with those interesting rocks, the ophites of the 

 Pyrenees f. Dr. A. Geikie has also described it as occurring in the 

 intrusive rocks of Carboniferous age in the central valley of Scotland $. 

 In this class of rock the augite is found crj^stallized in large patches, 



* Quart. Journ. Geol. Soc. vol. xxxii. (1877) p. 915, vol. xxxiv. (1879) 

 p. 783 ; Geol. Mag. dec. ii. vol. vi. (1879) p. 370. 



t Bvxll. Soc. Geol. d. Fr. 3rae ser. torn. vi. (1877) p. 156. 

 X Trans. Eojr. Soc. Edinb. vol. xxix. (1880) p. 495. 



