6 B. Hobson—Mezxican Volcanoes. 
but the most interesting point was the exposure in the vertical 
barranca-wall of a transverse section across a small stream-bed filled 
with pumiceous tuff differing slightly in colour and bedding-planes from 
the main mass, showing that water had played a part in the deposit of 
the almost horizontally bedded tuffs. Higher up we entered a pine 
forest, and climbed a shoulder whence a splendid view of the summit 
of the mountain and the surrounding country was obtained. After 
descending, with many windings, through most beautiful woods 
almost as much as we had ascended, we reached the Hacienda del 
Veladero, where we lunched. We then resumed our ascent, following 
the edge, crossing and then following the bottom of a deep barranca in 
volcanic tuff, until we reached our quarters for the night, two specially 
erected wooden sheds. Rising at 5 the next morning, after a 
somewhat sleepless night owing to the cold, we ascended through 
a pine forest amid wild lupines. The sun was rising, and through 
the beautiful foreground of pines we saw a magnificent view; the 
broad and flat plain to the east was filled with a sea of cloud, above 
which towered in the distance the snow-clad cone of Popocatepetl 
(17,876 feet) and the broader white summit of Ixtaccihuatl (17,318 
feet). Abruptly the forest came to an end at 4,100 metres (18,448 
feet), and we rode by a winding path on the treeless but grassy 
mountain side until we reached a cairn at one of the lowest points on 
the crater rim, whence we obtained a fine view of the interior of the 
crater with its central lava dome.' We rode down the interior slope 
of the crater to the shore of the Laguna Chica, the smaller of two 
lakes of crystal clear water, which is separated by the central lava 
dome from the Laguna Grande,? which is about 300 metres long by 
213 broad and 10 metres in maximum depth, according to D. Joaquin 
Velazquez de Leén. The lakes le at 4,270 metres (14,005 feet) 
above sea-level, hence the highest point of the crater rim, ‘‘ Pico del 
Fraile,” to which some of us climbed, rises 968 feet above their 
surface. The inner slopes of the crater have an average inclination 
of 380°, and are covered by screes to within a short distance of the 
rim: scattered patches of grass and wild flowers occur to a considerable 
height, above which are bare rock and scanty patches of snow. 
According to Dollfuss & Montserrat the elliptical crater measures 
along its major axis from north-west to south-east 1,431 metres 
(1,565 yards) by 595 metres (650 yards) along its minor axis. 
Mr. T. Flores regards the cone as built up of a series of lava-flows 
covered by pumiceous tuff and breccias. Some of the lava masses 
form prominent crags around the crater rim, but the tuffs form the 
main feature of the outer slopes of the cone, at any rate on the north 
and north-east sides. The final manifestation of volcanic energy was 
marked by the formation of the central lava dome. According to 
Mr. E. Ordénez all the lavas are hornblendic hypersthene andesites. 
The evidence as to the age of the volcano cited by Mr. Flores does 
not appear to me quite convincing. He says that Mr. J. G. Aguilera ® 
1 See Plate II, Fig. 2. 
2 See Plate II, Fig. 3. 
3 «* Bosquejo Geologico de Mexico’’: Boletines del Instituto Geolégico de Mexico, 
Nos. 4, 5, and 6, p. 229. 
