1884.] Microscopic Structure of some Rocks from Ecuador. 409 



I. IT. 



S.G-. 2-32. S.Q. 2-386. 



Si0 3 7776 77-01 



Alo6 3 13-14 .... 12-90 



Cab 0-63 .... 0-21 



MgO 0-29 



Fe 2 O 3 1-47 .... 1-83 



Alkalies and loss . 7 '00 7' 71 



100-00 100-00 



The silica percentage in the specimen which was examined for me 

 by Mr. J. J. H. Teall was not quite so high, being = 72'99 ; its specific 

 gravity was 2'337.* 



As the descriptions of these rocks from the Ecuadorian Andes have 

 had to appear separately, it may be convenient to conclude this series 

 of notes by a brief summary in a tabular form of the volcanic pro- 

 ducts of which I have described the microscopic structures. 



It will also serve to impress upon us the general uniformity of the 

 rocks occurring in the district. This was so obvious to Mr. Whymper, 

 that he deemed it needless to bring many specimens from the 

 mountains which he visited in the latter part of his journey. Yet the 

 area which they occupy is a large one. From Altar and Chimborazo, 

 the most southerly pair, which lie on an E.S.E. W.N.W. line, about 

 40 miles apart (passing by Carihuairazo) to Cotopaxi and Illiniza 

 (on nearly the same parallel of latitude) is rather more than 60 miles. 

 Next come Corazon, Sincholagua, and Antisana (also roughly on an 

 east and west line, the first being not much less than 40 miles from 

 the third). Passing northward by Pichincha (N.W. of Quito), we 

 leave on the east, Cayambe, which is a few miles east of the longitude 

 of Antisana, and finally reach Cotocachi, the most northerly volcanic 

 summit, having traversed a zone whose extreme length cannot be 

 less than 130 miles from N. to S., and whose greatest breadth from 

 east to west must be about 40 miles. f If, as some think, volcanic 

 rocks are but sedimentary deposits melted down, this general 

 uniformity is rather remarkable. 



* Zirkel (" Microsc. Beschaff," p. 448) says that he has not found free quartz 

 under the microscope in the so-called augite-andesites, but I do not understand 

 whether he includes among them the rocks mentioned by Professor Wolf as occurring 

 on the west side of Antisana. The composition and structure of the pitchstonea 

 described above make the occurrence of dacite in the highest degree probable, but I 

 have not myself noticed free quartz in any of the ordinary Andes rocks. I regret 

 that by an oversight "former" was written for " latter" at the end of the article 

 on Antisana. I consider the pitchstone of Urcucuy, as stated in the body of the 

 article, to be more probably allied to dacite than to rhyolite. 



t The mountains are thus grouped by Mr. Whymper (reckoning from north to 

 south) Western Andes: Cotocachi, Pichincha, Corazon, Illiniza, Carihuairazo, 

 Chimborazo. Eastern Andes : Cayambe, Antisana, Sincholagua, Cotopaxi, Altar. 



