Geology and Mineral Resources of Paraguay A Reconnaissance

56 GEOLOGY AND MINERAL RESOURCES OF PARAGUAYFIGURE 46. Typical joint pattern and pitted surface of the white sandstone unit of the Caacupe series near Tobati.as to form flutings and deep, almost vertically walledravines that give the outcrops the appearance of limestoneweathered in an arid climate (fig. 46). Along afew of the joints the sandstone contains much whiteand ocherous clay, possibly of hydrothermal origin,which forms the basis of a tiny industry at Tobati formanufacture of ocher, laundry bluing, and billiardchalk. (See figs. 47 and 55.)Except for the few clay-bearing localities just mentioned,the sandstone is made up of clean quartz sandwith a little calcareous cement and no other impuritiesexcept widely scattered fine grains of magnetite. Thefine quartz grains are uniform in size, but very irregularin shape, ranging from round grains to euhedralcrystals. The surface of the rock is commonly casehardenedto depths of 5 to 30 centimeters but its interioris saccharoidal. It may well be that the friablenature of the rock as seen in outcrops is a near-surfacefeature, caused by solution of the intergranular cement,and that the rock is better consolidated at depth.There are no available facts to confirm this possibility,however. Many of the surfaces, especially horizontalones, exhibit polyhedral cracks, resembling mudcracks(fig. 45), that seem to be as characteristic of the unitas are the weathered joints and pitted, limestonelike,weathered surfaces elsewhere.Upper shale and sandstone unit. The uppermost unitof the Caacupe series, about 200 meters thick, is evenless well known that the other units. Scattered informationsuggests that the lower half is made up largelyof soft shale or clay, with thin beds of sandstone,whereas the upper half is largely micaceous sandstonewith comparatively little clay or shale. The followingnotes, arranged stratigraphically, represent all the informationon this unit that is available.Just east of the town of Tobati the white saccharoidalsandstone unit dips beneath the surface of thebroad valley of the Rio Piribebuy. The underlyingbeds are covered almost everywhere by as much asC meters of river-deposited clay, silt, and sand. A fewlow forested hills, or "islas," and occasional outcrops inthe banks of the main stream serve to give some ideaof the older rocks. They consist of brown, fine-grainedcross-bedded sandstone, dipping eastward at low anglesand directly overlying the white saccharoidal sandstone.Despite the evidence of these exposures, the generalcharacter of the valley, as well as the rarity of exposuresmake it appear probable that the valley is carvedin a series of very soft shale or shaly sandstone beds,with only a few relatively thin beds of harder sandstone.Cerro Aparipi, from which Beder and Windhausen(1918) and later Harrington (1950) collected an exten-