14 MR. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. lxxvi, 



appearance from the smooth highly-polished surfaces of the fine- 

 grained blocks of basic lava that are found scattered in the vicinity. 

 In a hand-specimen the rock bears a strong resemblance to the 

 well-known syenite of the Plauens'cher Grand, near Dresden, 

 being composed almost entirely of a dull pink felspar and greenish- 

 black hornblende ; in microscopic section, however, it appears to be 

 more closely related to the acid end of the monzonite series. 



(A 75) Quartz-monzonite. Vitor. 



Microscopic characters : — This rock is composed mainly of felspar and 

 uralitic hornblende, with subsidiary quartz, biotite. and minor accessories. 



It has a coarse-grained holocrystalline structure, and, with the excep- 

 tion of containing a less amount of biotite, agrees in all essential features 

 with the typical monzonite of the Tyrol. 



The plagioclase (oligoclase-andesine) is remarkably free from altera- 

 tion, although traversed by numerous irregular cracks ; it is present in 

 slight excess of the orthoclase, to which it' tends to be idiomorphic : the 

 latter is grey and somewhat turbid. 



The hornblende occurs in the form of yellowish-green fibrous para- 

 morphs of uralite after augite, which was probably a variety rich in iron, 

 for the uralite is crowded with disseminated grains of magnetite and 

 haematite. Indications of dual and multiple orthopinacoidal twinning in 

 the original crystals are still visible. 



Quartz is fairly abundant : it appears somewhat dusty, owing to the 

 presence of numerous liquid inclusions which frequently show mobile 

 biibbles. 



There is a small amount of reddish-brown biotite showing alteration 

 into chlorite, and the felspar includes numerous minute needles of apatite. 



After leaving Vitor (kil. 123) we commence the ascent of the 

 ■Cerros de la Caldera, and at once meet with a complex suite of 

 igneous rocks, representing, as mentioned above, at least three dis- 

 tinct phases of deep-seated plutonic intrusion. Many of these 

 rocks have been subjected to powerful earth-movements with 

 resultant dynamic metamorphism, and the development of banded 

 or gneissic structure. The gradual passage of a coarsely-crystalline 

 granitoid plutonic rock into a fine-grained gneiss can be clearly 

 followed at more than one locality. 



Becrystallization of many of the component minerals is a 

 further well-marked feature : but it is not easy to determine how 

 much of this is due to dynamic agency, and how much to thermal 

 metamorphism of one member of the series by another. 



Before I attempt to describe the minute structure of the rocks 

 forming this complex, it will be as well to run briefly over the 

 section, stating the various types met with and the evidence on 

 which a determination of their sequence has been arrived at. The 

 majority of the specimens in my collection were obtained from the 

 actual railway-cuttings, where the exposures were comparatively 

 fresh, and as the line ascends with the most sinuous curves, it is 

 possible that intrusions of the same rock, regarded as separate, 

 might in some cases prove to be in reality connected. This, how- 

 ever, is a point of minor importance in reading the general history 

 of the complex, and the detailed mapping, which alone could have 

 made this clear, would not have repaid the time necessary for the 



