284 
GEOLOGY: E. W. BERRY 
times the evidence is accumulating that their final elevation was only 
accomplished in the late Tertiary or even in the Pleistocene and there 
is some evidence that this upward movement is still going on. Thus 
Steinmann considers the dioritic rocks of the copper belt as of late Ter- 
tiary age and Stille^ states that the upHft of the eastern Cordilleras in 
Colombia must have been in the late Tertiary because of the part taken 
in the movement by the Miocene Honda beds. Neither Engelhardt nor 
Britton in their studies of the fossil plants from Potosi venture beyond 
Tertiary in their age determinations. In my own work I have always 
regarded this flora as probably of PHocene age because of its resemblance 
to the existing flora in this general region. The collections made by 
Singewald and Miller but emphasize the opinion that the flora is very 
young and I am not yet sure that it will not eventually have to be con- 
sidered Pleistocene. 
More spectacular than the floral evidence is that of the fauna which 
is found in the same series of tufaceous materials at a somewhat lower 
level. The only determinable form is a new species of Discinisca which 
has been determined by Professor Schuchert, who will describe it in the 
final report on this region. Professor Schuchert states that this form 
is related to the existing Discinisca lamellosa (Broderip) which is found 
in shallow water along the west coast of South America from Panama 
to Chile, and that it cannot be older than Miocene and may be Pliocene 
or Pleistocene. 
The extreme youthfulness of these beds indicated by the Brachiopod 
and confirmed by the more extensive evidence furnished by the flora 
shows that the sea deposited a part of these strata in late Tertiary or 
Pleistocene time and since that time there has been differential vertical 
movements amounting to a minimum of 13,500 feet. 
The fossil plants denote a much more humid climate than prevails 
today in this region. For example at Corocoro which now lies at a Httle 
over 13,000 feet above sea level, the country is practically a treeless 
desert. The fossil plants from this locality include a fern (Polystichum), 
fruits of Terminalia and Copaifera, both tropical trees of the eastern 
subandean hills and Amazon Basin; leaves of Mimosa arcuatifolia Engel- 
hardt, Mimosites linearis Engelhardt, Acacia uninervifolia Engelhardt, 
and Cassia ligustrinoides Engelhardt, the last four common to Potosi 
and suggestive of the existing deciduous forests of the so-called Pan- 
tanales region of the eastern plains of Bolivia. 
If the moisture carrying winds were from the east at that time as they 
are at the present time, the lowering of the eastern Andes would enable 
such a flora to flourish in the present inter montane region. 
