circumstance has caused, that a skarn (silicatic) mineralization very 
4 
CRYSTALLINE BASEMENT 15 
4 ‘The limestone is characterized by a high grade of purity. This 
‘seldom occurs, in spite of the abundant, crossing granitic veins. In 
some parts there may be found a hornstone, a characteristic mark of 
‘the precordilleran silurian limestone, and formed probably from 
remnants of silica-bearing organisms (radiolaria). The mightier layers 
of the limestone are generally divided into thinner banks by joint 
; planes, seldom by intercalations of foreign matter. The banking 
of the rock marks very well the folding and torsion phenomena. 
In handspecimen the rock shows generally an evengrained structure 
- (sugargrained) and the colour is mostly pure white. In some parts 
there are very excellent kinds of marble. An analysis of the lime- 
stone of usual habit from the Cerro Potrero Viejo, near to the 
Puesto del Agua, was obtained from the Chemical Laboratory 
of the Ministery of Agriculture, Section Mineral Waters, Buenos 
Aires: 
SION A ESRI UME at tay 4,65 /o 
BO A N ER 41,09 °/o 
OE (OPT le Ae N N 35,30 °/o 
TRAN GH Ga a pena aia ee 18,04 2/0 
NE OA OUI N la ö 
SAO VN SA ates ra ESS | hou a 
Total 98,21 °/o 
From this analysis it is evident that the rock contains a consi- 
derable amount of MgO, but not to such a degree, as to be named 
dolomite. 
Other crystalline limestones from the area of the pampean 
Sierras, i. e. the quarry Rio del Sauce, Sierra de Cordoba, whose 
analysis-data were communicated by my colleague Dr R. Beder, 
show scarcely more than traces of MgO in eleven limestones ana- 
lysed. Over all the Cördoba-limestones (Malagiienio, Santo Do- 
