1242 
proves that the Serra do Gericino constitutes the strongly denuded 
remnant of a voleano or group of volcanoes, like the Serra de Tingua 
further northward. That remains of lava-flows have been preserved 
only locally, and only between the eruptive rocks, whereas they 
do not occur in the surrounding gneisses, points to the circum- 
stance that these effusive rocks, which originally must have extended 
far beyond the present mountain, have long been protected from 
erosion through overhead stoping, the roof having locally sunk 
down. These effusive rocks occur near the station of Maxambomba 
of the E.F. Central do Brazil, and near the facenda D. Eugenia 
close to the west of this station. 
Dyke-rocks were e.g. met with near the station of Maxambomba 
(tinguaite), near the facenda Mascarenhas and in the western part 
of the eruptive province, between Cava and Ypiranga (aegerine- and 
amphibole-Sölvsbergite). | 
The coarse-grained rocks, which chiefly compose the eruptive 
province, are generally characterised by table-shaped felspars; con- 
sequently they belong to the foyaites as far as they contain nepheline. 
Of the western part, known as Serra de Marapieu, samples of 
nepheline-free umptekite were examined, while among the foyaites, 
which seem to build up the greater part of the mountain-ridge 
between the Serra de Marapicu and the station of Maxambomba, 
also alkali-syenites (partly pulaskites) occur ') 
The granular rocks. 
The following types may be distinguished : 
1. Foyaites. 
2. Alkali-syenites. 
3. Umptekites. 
4. Pulaskites. 
Foyaites. 
They seem to be the most common rocks of the Serra do Gericino. 
Type 1 is a pyroxene-amphibolafoyaite, collected near Cancella 
Azul along the road which crosses the rivulet Cachoeira. The com- 
posing minerals are light-coloured orthoclase and microperthite, 
‘) Most of the samples received, which had been provisionally determined as 
nepheline-syenite, belonged to the alkali-syenites. The typical foyaites originate 
from Cancella Azul along the road intersecting the rivulet Cachoeira. Therefore, 
because many of the rocks have apparently been mistaken for nepheline-syenites, 
whereas they are actually alkali-syenites, the data occurring on a map on which 
different types of nepheline-syenites have been separated cannot be relied on. 
