206 FIELD COLUMBIAN MusEUuM—GEOLOGY, VoL. II. 

Quantitative Composition as Composition of 
composition. affected by kaolinite unaltered rock: 
and limonite. 
Oud its ce eee ns ee 1992 16.58 15-30 
Orthoclase.<. ieaa toe E2528 £2.23 12494 
"ALD TGET hie tang acem  aogee te 49.26 49.26 60.60 
AMOnE nites’; 4 Malini oe ete aes Pelli 7.51 9..25 
Goriindium: ! i. eee otek 2480) 8 Do. eens ae 
Kadlinitet ye ae eee ees 920.) See 
Hypersthenes 0 ae se eM 4s: 3.26 2x20 
Hematite) A tana ee 2.87 2.04 2.87 
LAmonitee..0 ae eee prac 0.97. ieee 
W ater in Aiea 2 hee TTA, ~ bebitoe 2 Sey Ser 
SO. 4s honk cee eens 0.87 0.87 0.87 
99.92 99.92 100.47 
From the composition given in the first column, the classifica- 
tion would be Class 1, Persalane, Order 4, Brittanare, Rang 2, Dom- 
alkalic, and Subrang 4, Dosodic, Lassenose. 
The composition as given in the third column would change the 
position of the rock in classification slightly, putting it in Order 5, 
Canadare, and making it Laurvikose. 
About two hours from the Ciudad de Rocas, at one point on the 
trail, a considerable quantity of black obsidian in the form of scat- 
tered fragments was seen. The fragments were observed over an area 
several rods in extent, but no outcrop of the rock in place was visible 
in the immediate vicinity. The fragments ranged in size from pieces 
as large as one’s fist downward. They were generally more or less 
rounded, but showed no evidence of artificial flaking. As the number 
of known occurrences of obsidian in Mexico is very few, this one is 
of interest to add to,the list, and it 1s also of importance as illustrat- 
ing the igneous nature of the rocks which cap the surface of the 
plateau. 7 
A few hours west of the Ciudad ranch the way began to descend, 
and soon the western edge of the plateau was reached, where the 
mountains of the Pacific slope came into full view. As far as the 
eye could sweep, north, south, and west, a maze of peaks and val- 
leys appeared, forming a veritable sea of mountains. (Plates LX1L., 
LXII., and LXIII.) The summits of these did not appear to be el- 
evated above the plateau, but to reach approximately to its level. They 
evidently originated from a profound dissection of the edge of the 
plateau by swift, short rivers, since they tend to run in ridges trans- 
verse to its border. The peaks in form are sharp and angular, and 
they are separated by deep, steep-walled valleys. Bold cliffs are com- 
mon, and the waste slopes are steep and largely barren of vegetation. 
The valleys nearest the plateau often have a well-marked amphitheatre- 
like form, as illustrated by the one shown in Plate LX., and a general 
