A phonolite plug forms a steep, conical hill about 1 km in diameter. Little detail is known.
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A brief review of occurrences of alkaline igneous rocks in Paraguay, including some petrography, is to be found in Putzer (1962). A similar account is given by Milton and Eckel (1959) with fuller details for some localities, but the coloured geological map of Paraguay accompanying this paper, on which many alkaline intrusions are shown, is not reliable in many instances
A horseshoe-shaped hill, open to the south, is shown as alkaline on Plate 1 of Milton and Eckel (1959), but no other information is available.
A small hill 3 km north of Acahay (No. 16) is indicated as being alkaline on Plate 1 of Milton and Eckel (1959). No further information is available.
Nepheline syenite. No details are known.
This is a nepheline syenite intrusion spanning the Paraguay River and having a more extensive outcrop in Brazil. For fuller details, including age and references, see Brazil No. 70.
This locality consists of nepheline syenite. It is shown as two hills on the map of Milton and Eckel (1959, Plate 1), though not indicated as being alkaline; it is referred to by them as Porto Murtinho. No details known.
This is probably a complex of dykes, but shown as a major intrusion by Milton and Eckel (1959, Plate 1).
A phonolite plug 300 m in diameter.
Chiriguelo is an approximately circular complex about 6 km in diameter.
This complex forms an approximately circular structure about 5 km in diameter emplaced in Precambrian metamorphic rocks which are domed and surrounded by Silurian and Permo-Carboniferous sedimentary rocks.