172 M. Delesse on Globuliferous Rocks. 
There are irregular exterior zones of feldspar of different shades, 
u; needles or conoids of pe 3 converging from the zones 
towards the centre, and others diverging from the centre out- 
ward, the two sets much interlaced; the forms and structure of 
these conoids are very irregular, and sometimes they become iso- 
lated globules. Quartz penetrates the whole, and is also to a 
considerable extent free and glassy (6), filling the spaces or form- 
ing zones. Sometimes’ the quartz is much more abundant at the 
centre than at the circumference. The quartz is beautifully 
brought out by means of hydrofluoric acid; but t should. be 
observed that hyaline a i is much less readily oucied than 
chalcedony or opaque qua Ng 
Abnormal globules. Tes e globules contain cavities, which 
may be reguiar or confused, filled or unfilled ; and they have been 
produced either by contraction or expansion. Globules with eavi- 
ties formed by contraction are common in the trachyte and other 
rocks of Iceland. One of these enlarged is shown in fig. 5. The 
cavities are of irregular form and surface. Sometimes they” have 
been filled with quartz. and occasionally with 5. 
oxyd of manganese, spathic iron, zeolites, ae 
rytes, calcite, etc. At Llan beris m Wales, 
the globules are sometimes a decimeter in 
an exterior zone of opal and an interior of 
jad crystals, or a kind of jasper in layers 
r bands. 
In other globules the cavities are numerous and confused; and 
often they have been filled with quartz, which may be quite dis- 
tinct from the feldspar or shade into it. In Corsica, the cavities 
The indent inate of globules, occurring in the feld- 
spathic paste constituting them, may be either quartz, ortho- 
The most ee PRN of the rocks containing 
globules, is their holding an excess of silica; and in a given rock 
they are especially developed where the silica is most prevalent. 
By this presence of silica, connected in some cases with the shi 
etration of the rock by veins of quartz, the occurrence 
formation of the globules has been determined. Mc 
the globuliferous rocks are ae eecepel nd the 
