1884.] Microscopic Structure of some Rocks from Ecuador. 403 



it must undoubtedly be classed with the crystalline schists, but I 

 suspect that the very marked schistosity is a secondary development 

 due to crushing. 



From the debris on the peak of Sara-urcu one specimen has been 

 examined. It is a fairly fissile mica-schist consisting mainly of a 

 silvery mica in rather small flakes, with apparently a quartzose or fels- 

 pathic constituent. Under the microscope there appear to be two micas 

 one, far the most abundant, a rather light yellowish-green colour, the 

 other colourless. There is a moderate amount of rather clear quartz, 

 containing occasional small microliths, probably of the former mica, 

 and very minute cavities. There is some calcite, which often includes 

 a minutely granular earthy-looking material rather clustered together. 

 The layers of mica exhibit a kind of foliation, but the actual flakes are 

 often (though not always) athwart the layers ; this, together with 

 other indications in the slide, leads me to suspect that the rock sub- 

 sequently to its foliation has been compressed in a direction making a 

 considerable angle with the original foliation planes. 



Another specimen is a rather friable schist, consisting chiefly of a 

 dark green, or nearly black mica, with a finely granular green mineral, 

 which I have little doubt is epidote ; there is also a little of a silvery 

 mica. A third specimen is a piece of a quartz vein, from a rock 

 identical with the last mentioned. Three other specimens from Sara- 

 urcu, one broken from rock in situ on the summit, were examined 

 macroscopically by myself soon after Mr. Whymper's return, but have 

 since been unfortunately mislaid. The summit rock appeared to me 

 a rather fine-grained gneiss, containing quartz, felspar, dark mica, 

 with probably a little chlorite and epidote. Another was a schist, in 

 which I distinguished quartz, dark mica, and a silvery micaceous 

 mineral resembling sericite. The third was a vein specimen from a 

 ' rock corresponding with the last named. I remarked in a letter to 

 Mr. Whymper at that time : " These specimens prove the existence 

 of true metamorphic rocks, which are in about the same condition as 

 many of those in what is called the ' newer gneiss ' series of the Scotch 

 Highlands." 



A specimen from the moraine on the west side of Sara-urcu has been 

 examined microscopically. It is a dull-coloured rock, which consists 

 of a micaceous mineral with silvery lustre, and a dark mineral in 

 small grains, being evidently a rather minutely constituted member of 

 the group of the spotted schists (fruchtschiefer). The lustrous aspect 

 is visible on the faces of the rough divisional planes ; on cross frac- 

 tures there is little lustre, and the rock has a slightly friable aspect. 

 Under the microscope, what we may call the ground-mass of the rock 

 consists of films of a very pale brown, almost colourless micaceous 

 mineral, felted together in wavy folia, and interspersed with numerous 

 specks of ferrite and occasional small grannies of quartz ; in this are 



