: 
M. Delesse on Gilobuliferous Rocks. | 175 
ble, and we cannot conceive of any other volatile ingredient being 
concerned. It is hence altogether probable that these abnormal 
globules at first contained silica in the state of a hydrosilicate, or 
ajelly rich in silica ; the water was at first retained by the pressure 
or the heat causing the spheroidal state ; and afterwards it gradu- 
ally escaped, and thus occasioned the contraction and the forma- 
tion of the cavities. 
In abnormal globules having cavities formed by expansion, 
these cavities have beyond doubt been produced by a disengage- 
ment of volatile substances, which has taken place while the rocks 
Were still fluid. When the volatile substances were simply gas- 
eous, they formed no deposit in the cellules ; but when they car- 
ried along other substances, cooling would lead toa crystallization 
of different minerals on the walls of the cellules. These miner- 
als are those composing the rocks, especially feldspar, quartz and 
mica: and in fact it has been found at Sangerhausen that feld- 
Spar may be deposited by sublimation in furnaces. 
In some cases, there has been an expansion by gaseous substan- 
Ces, producing an enveloping cellule, and afterwards a contraction 
forming a feldspathic globule at the centre of the cavity. In 
other cases, the expansior’ has taken place within the globule 
while the paste was still soft, producing a multitude of small cel- 
lules. These cellules have been afterwards filled by chalcedony. 
Filling of the cavities.—The filling of the cavities has taken 
Place either by secretion, infiltration or by both combined. In 
the filling of septaria, there may be a secretion of carbonate of 
ime or iron along the walls of the fissures, and alsoan infiltration 
filling up the rest of the cavities, the two processes producing de- 
Posits of different kinds or colors. In the filling of a mineral. 
Yein also, both processes have often operated, secretion introduc- 
ing a part of the minerals, and infilération another portion. 
The infiltration of the silica into cavities, may have been 
pe which is easily dissolved in hot water, when this water Is 
ider pressure and is charged with carbonic acid. 
whi € same causes account for the threads or veins of quartz 
ich intersect the globules. 
-elinj @ feldspar of the globules may be either orthoclase or a tri- 
are le feldspar. The abnormal globules rather than the normal, 
Whee veloped in globuliferous rocks, which are rich in soda. 
N orthoclase and a soda feldspar occur together, the latter is in 
1859.) that has recognized in a recent paper, (Rendiconto della R. Acad., — 
mica, quart t, hornblende, pyroxene, sodalite, nepheline and ot 
licates may form by sublimation. i = 
i, 
su 
have resulted from the penetration of the cavities with gelatinous 
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