1884.] Microscopic Structure of some JRocks from the Andes. 241 



II. " Notes on the Microscopic Structure of some Rocks from 

 the Andes of Ecuador, collected by E. Whyrnper, F.R.G.S. 

 No. I. Pichincha." By Professor T. G. Bonnet, D.Sc, 

 F.R.S. Received January 25, 1884. 



During his adventurous journey in the Andes of Ecuador in Decem- 

 ber, 1879, and the earlier months of 1880, Mr. E. Whymper succeeded 

 in ascending several of the highest mountains, and collected a con- 

 siderable series of rock specimens, which he has entrusted to me for 

 examination. Although descriptions of rocks from this district have 

 already been published by some eminent penologists on the Continent, 

 yet, as Mr. Whymper's collections were made with great care, and 

 specimens were taken in numerous instances from localities never 

 before touched by the hand of man, I have thought it better to make 

 my notes as far as possible complete, and so have occasionally included 

 specimens of which some account may have appeared in foreign 

 scientific periodicals. 



Probably no collection has hitherto been described which either has 

 been so extensive or has consisted of specimens with the localities 

 more exactly determined. 



In making these notes I think it better to group the specimens 

 geographically rather than mineralogically, so as to give successively 

 a synoptic view of the products of each volcano, and I select as the 

 subject of the first of a short series of papers the rocks from Pichincha, 

 although from its proximity to Quito, and its accessibility, it is one of 

 the best known of the Ecuadorian Andes, because the collection from 

 this mountain is a comparatively large one, and contains a considerable 

 number of well-preserved specimens. 



Pichincha, according to Mr. Whymper, is a dormant and nearly 

 extinct volcano, lying to the north-west of Quito, which city is built 

 almost on the extremities of its eastern slopes. To quote his words, 

 " From the south, north, and east (I have not seen the mountain from 

 the west) Pichincha has the appearance of a range rather than a cone. 

 It rises into several well-marked, but not bold, peaks, the principal of 

 which are Guagua- Pichincha, the highest point, which by my baro- 

 metrical-mercury observations is 15,918 feet (15,706 Reiss and 

 Stiibel A), and Rucu-Pichincha, which is only a few hundred feet lower. 



"The summits of the mountains Pichincha, Atacatzo, and Corazon 

 lie almost in a direct line, and Illiniza* is only slightly out of it. 



* Atacatzo, 14,892 feet (Eeiss and Stiibel, mere. bar.). Corazon, 15,871 feet 

 (Whymper, mere. bar.). Illiniza, 17,405 feet (Reiss and Stiibel, A). Quito 

 (Central Plaza), 9,363 feet (Whymper, mere, bar., mean of many observations) ; 

 Quito (uncertain in what part of city) ; 9,350 feet (Reiss and Stiibel, mere, bar., 

 mean of many observations). 



