RAE BROPTIVE ROCKS (OF MEXTCO 473 
tral, where some of them may be considered of later age than 
those of the Sierra Madre. 
In general it can be said that the forms of the mountains 
of rhyolites are always characteristic, serving in many cases to 
foretell their nature, especially when this rock occurs alone in 
an extensive portion of one serrania. Of those with sharp and 
elongated peaks we have a good example in the peak of Bernal 
in the state of Queretaro. The extended forms present us great 
cliffs (acantilados) in the extensive serrania of Valdecanas and 
in the no less interesting Sierra Fria in the state of Zacatecas. 
In the second of these serranias, formed in great part of 
rhyolites, variations of structure and texture have brought about 
the formation of plateaus and dome-like summits and erosion 
has given rise to broken, fantastic shapes. In this sierra as in 
many other localities formed of the same rock, the spherulitic 
rhyolites of slightly coherent or tufaceous paste alternate in-beds. 
more or less horizontal or parallel with petrosiliceous rhyolites 
charged with quartz, which are compact and resist the forces. 
of erosion. The result is an appearance of steps or stairs at 
different heights on the slopes of the mountains. The surfaces 
of the separated blocks, as a result of contraction or atmospheric 
action, generally have columnar or other imitative forms, such 
as are shown by some of the peaks called The Friars; a name 
by which rocks of columnar structure are designated in various 
parts of the country. 
As notably spherulitic rhyolites can be mentioned those of 
Chichindaro, in the state of Guanajuato, and those of San 
Ildefonso, Tula, Hidalgo. There are spherulitic and _perlitic 
retinites in Apaseo el Alto, which are of pretty appearance on 
account of the contrast of color which the gray or black amor- 
phous paste offers to the generally red spherulitic globules. But 
the most abundant rhyolites are the petro-siliceous rhyolites of 
various shades, red, black, violet, etc., such as those of the 
Sierra del Jaral and other points in the vicinity of the regions 
of San Luis Potosi, together with those of Guanajuato, Pozos, 
Pefioles, etc. Some of the latter frequently are accompanied 
