304 TJic American Geologist. May, i898 
This formation, both in appearance and in locaHty, very 
stronfjly reminds one of the nagel-flue formation that dis- 
tinguishes the first descent of the ice in the Alps, and it would 
seem highly probable that the mode of origin in Patagonia 
is the same, though there the phenomenon has been of much 
greater extent. 
The boulder formation is most extensive in Central Pata- 
gonia : in the southernmost part of the continent and in Terra 
del Fuego it is of much less importance. In those parts it 
is replaced by another not less interesting formation that can 
be very conveniently investigated at many points on the east 
coast of Terra del Fuego, for instance at cape San Sebastian. 
The "barranca," over sixty metres in hight, consists through- 
out of an entirely unstratified clay containing, in the utmost 
disorder, masses of angular-shaped stones varying in size from 
the smallest imaginable to great blocks of a volume of several 
cubic metres. Among these stones there are many that dis- 
play traces of glacial polish and stria. Fossils are not found* 
with the exception of occasional broken shells of a furritella, 
common in the underlying Tertiary deposits ; these are pre- 
sumably not original here. It cannot be questioned that this 
boulder clay is formed in the same manner as the correspond- 
ing deposits in the northern hemisphere, and that it consti- 
tutes the ground moraine of a thick layer of land-ice. f 
On the coast further north numerous irregular, often len- 
ticular intercalations of gravel and sand occur. These do not 
contain either any traces of fossil remains. They are notice- 
able, however, for a cross-bedding, extremely usual and plainly 
to be seen. This is characteristic of river-glacial deposits, 
a-nd the sand is frequently permeated with small faults. Both 
above and l^elow the boulder clay irregular layers of sand 
and gravel are often found. Still further north the bouk'er 
clay itself forms an intercalation in a thick shingle formation 
on the plateau ; it often takes the form of two or more narrow 
somewhat irregular layers, one above the other. 
This boulder-clay gives origin to a peculiar form of land- 
*Not even Diatomacea? or other microscopic organisms. 
tOf the blocks the majority consist of rock varieties from the Cor- 
dilleras; of these many have proved under the microscope to be identical 
with the white eranite from the Western islands. 
