12 B. Hobson—Mezxican Volcanoes. 
The hornitos do not agree with Humboldt’s description, part of which 
is as follows: ‘‘ Hach of the numerous Hornitos consists of weathered 
balls of basalt, with pieces separated in concentric shells; I could 
often count 24 to 28 such shells. The balls are somewhat spheroidally 
flattened, and are mostly 15 to 18 inches in diameter; but also vary 
from 1 to 3 feet . . . .! Yhe soft mass of clay which unites the 
balls is, strangely enough, divided into curved lamelle, which wind 
about through all the interstices of the balls.” In this description 
Humboldt appears to have been in error. 
Petrography. 
As to the petrographical nature of the lavas of Jorullo, according to 
Ordénez they are olivine basalts, and there is no appreciable difference 
between the successive lava-streams. Specimens collected by me from 
the top of the main crater rim, the surface of the termination of lava- 
stream No. iv, and the lapilli of the main cone agree in essentials. 
Phenocrysts. Olivine is present in all cases ; augite, almost colourless 
in thin section, is less common ; plagioclase (contrary to Mr. Orddiiez’s 
observation ‘‘une premiére génération de teldspath manque presque 
compléetement’’) occurs in numerous small phenocrysts (except in the 
lapilli). Brown hornblende crystals, with a dark resorption border, 
occur like phenocrysts (but may be inclusions). It was absent from 
the lapuilli. 
The groundmass makes up the bulk of the rock, and consists of 
small but idiomorphic crystals, viz., felspar laths, augite crystals, 
olivine, often reddish brown, and specks of black iron ore, all embedded 
in a brownish globulitic glass filling the interstices between the felspar 
laths and most abundant in the lapilli. 
The older basalts of the plateau of Higuera and the older volcanic 
cones are distinguished from those of Jorullo by the presence in them 
of nepheline and hypersthene. 
After examining the hornitos on August 29th we made our way to 
the Hacienda La Playa, whence we rode to Ario, and on the 30th to 
Patzcuaro, which we left on the 31st by train for Mexico, arriving 
there at 6.30 a.m. on September 1st. 
Humboldt’s Craters of Elevation. 
Humboldt* considered Jorullo an example of one of Von Buch’s 
craters of elevation due to the swelling up of a pre-existing surface. 
Of this there is not the slightest evidence. Mr. Ordénez* thinks the 
eye-witnesses of the welling up of the earlier lava-streams of Jorullo 
were misled by them into thinking that the ground had risen in 
swellings. 
The possible existence of a Line of Fissure. 
Mr. Ordoiiez, while not rejecting a priord* the hypothesis that the 
four cones of the Jorullo group are on a line of fissure, does not favour 
1 T translate from the German edition of ‘‘ Kosmos,” vol. iv, pp. 340, 341 (J. G. 
Cotta’scher Verlag, Stuttgart, 1858); English version (Bohn’s, 1865), vol. v, p. 317. 
2 “ Kosmos,”’ vol. iv, pp. 840, 336. 
Y ILGG, City, 10s 42. 
YEG, Chior, Ds LL 
