376 PROCEEDINGS OF THE ACADEMY OF [1898. 



ola are found growing here as saline plants. The chinampas are 

 the so-called floating islands, more especially found in Lake Xochi- 

 milcho. 



A number of small conical volcanoes rise from the floor of the 

 valley, and are known locally as Cerro de taza (cupped hill, Kup- 

 pen). One remarkable collection of these low hills is to be found 

 in the neighborhood of the town of Guadalupe, separating the Val- 

 ley of Mexico into a northern and southern portion. These, the so- 

 called Sierra de Guadalupe, are connected with the western mount- 

 ain chain Monte bajo, by the low ridge Cuesta de Barrientos, and 

 with the north-eastern ridges by the Cerro de Chiconautla. The 

 northern half of the valley thus formed is occupied by Lakes Zum- 

 pango, Xaltocan and San Cristobal, the southern half by the three 

 lakes best known to travellers, Texcoco, Chalco and Xochimilcho. 



A few words are necessary as to the geography, topography and 

 geology of the region visited botanically. Cerro de Tepeyac, one of 

 the hills of the aforementioned Sierra de Guadalupe, is 140 ft. high ; 

 very few plants are found on this hill. If one ascends the east side, 

 he finds for the first 115 ft. a fine crystalline rock of a dark violet- 

 gray color of a close texture frequently spotted with green, scaly, 

 porous particles. The summit of the hill is covered with a pitch- 

 stone-like rock formation about 16 to 26 ft. thick. By a rocky bridge, 

 Cerro de Tepeyac is connected with the Cerro Gachupines. Felix 

 and Lenk 2 say of this hill : " An dessen Abhang passirt man zun- 

 acbst zwei, 5 bezw. 8 m. machtige Pechsteinzonen, zwischen und 

 uber welchen der rothlichgraue.kornige Andesit sich ausbreitet, der 

 die Hauptmasse des Berges ausmacht und in einigen Steinbriichen 

 zu industrielleu Zwecken abgebaut wird." 



The ped regal of Tlalpam, or of San Angel is one of the most 

 interesting formations in the valley, covering an area of about 

 7,000 acres. The pedregal is an extinct lava stream situated be- 

 tween the towns of San Angel and Tlalpam, and extending south- 

 ward up the sides of the Sierra del Ajusco to the hill called Chitle. 

 It was formed when the southern mountains were in active volcanic 

 eruption. The eye of the traveller sweeps unobstructedly over an 

 arid black landscape, which might be compared to the sudden stif- 

 fening of the rolling surface of the sea. The country is extremely 

 uneven and rugged ; the coulee of lava is full of cracks, blisters, 



2 1890. Felix and Lenk, Beitrilge zur Geologie und PaUiontologie der 

 Republik Mexico, I, 70. 



