GABBROS ETC. IN SCOTLAND AND IKELAND. 95 



effects of this action are especially well seen in the augite crystal, 

 which, by schillerization, is partially converted into diallage. These 

 radiating series of cracks proceeding from the olivine grains, and in- 

 jected with serpentine, are generally very well seen in the troctolites 

 (" Forellensteine ") among the older rocks, where the olivines are more 

 or less perfectly serpentinized (see p. 86). In the Tertiary rocks it can 

 frequently be recognized, and in some of the more altered gabbros, like 

 those of Mull and Skje, all the minerals of the rocks have their cracks 

 thus injected with serpentine. 



Fig. 8. Micropegmatitic (?) structure in the plagioclase felspar of the altered 

 gabbro of the Isle of Mull, seen as magnified 30 diameters. Compare 

 this with the admirable figure given in R. Breon's ' Geologie de I'ls- 

 lande,' plate vi. fig. 2. The manner in which this structure is seen to 

 be developed in the outer zones of some felspar crystals, and especially 

 in those Avhich fill up the interspaces between other earlier-formed 

 and well-defijied crystals, and the way in which they are constantly 

 associated with undoubtedly secondary quartz, all point to the con- 

 clusion that this structure is not an original, but a secondary one (see 

 page 72). As an illustration of the commencement of these secondary 

 structures in felspars the next drawing, fig. 9, is given. In the group 

 of crystals represented in fig. 8 we find several plagioclase crystals, 

 one of which encloses a granule of garnet, while the outer zone of 

 another exhibits the " micropegmatitic " structure. The same struc- 

 ture is seen in the surrounding ci'ystals. The clear material appears 

 to be quartz. The cloudy matter between may be kaolin (?). Infil- 

 trated serpentinous matter, with clear secondai-y quartz and a crystal 

 of augite schillerized and converted into diallage on its outer margin, 

 is seen in the same group. 



Kg. 9. Incipient stage of a secondary structure seen to be developed ' in the 

 felspars of a gabbro, from Portsoy, Scotland, magnified 180 diameters. 

 (A similar structure has, on moi'e than one occasion, been mistaken 

 for the canal- system of Eozoon !) Certain isolated portions of the 

 felspar crystals appear to be attacked in this way, and every stage of 

 the process may be studied in different specimens, from the incipient 

 appearance represented in the figure to a structure closely resembling 

 the so-called micropegmatitic shown in fig. 8. I have not been able to 

 determine the mineral substance which forms these radiating and 

 plumose masses which penetrate certain portions of the twinned felspar 

 (see page 73). 



Discussion. 



The Chathman congratulated the author on his paper, and said 

 that he had no doubt the evidence of the fossil plants from the 

 associated leaf-beds would prove the rocks referred to to be of Older 

 Tertiary date. Mr. J. Starkie Gardner had procured very extensive 

 series of these remains, and was now engaged in working them out. 



Mr. RuTLEY was very glad to find that the author objected so 

 strongly to the attempt at classifying igneous rocks according to 

 age. He thought that the term " diabase " might be dropped. The 

 structures represented in the diagrams were quite familiar ; but the 

 terms "granitic," "ophitic," &c., might perhaps with advantage 

 be replaced by " coarse-grained," " medium-grained," and " fine- 

 grained," the differences existing only in structure and not in con- 

 stitution. He alluded also to the diiferent phases of " micro- 

 crystalline " structure. A most interesting point was the distinction 

 between coarse-grained deep-seated rocks, less coarsely crystalhne, 

 and, finally, vitreous conditions of basalt, near the surface ; in this 



