266 FIELD CoLUMBIAN MusSEUM—GEOLOGY, VOL. I. 
affords a rule perhaps of some value as a guide to the conditions 
under which banded formations have taken place as compared with 
those which exhibit distinct crystals. Substances deposited from 
solution in water which exhibit a banded or layered structure have, 
according to this rule, been formed by moving waters, while those in 
the form of distinct crystals have been deposited from waters at rest. 
Hence, the banded structure so characteristic of mineral veins may 
be considered proof that the deposit was formed from moving waters 
while the occasional cavities lined with crystals show points at which 
the solutions were at rest. Similar conclusions may be drawn 
regarding the same structures as seen in agates and geodes. It is 
evident, further, that the conditions in the two cases also differ 
in the quantity of liquid present and inthe rate of deposition. The 
layered structure is the result of ¢véck/ing waters from which deposi- 
tion is necessarily rapid, while the distinct crystals were formed from 
a solution which was present in quantity, and from which deposition 
was comparatively slow. The applications of these principles to 
conclusions regarding the origin of veins are obvious. The terms 
motion and rest are, of course, here to be understood in a purely 
relative sense, as no body .of liquid would be entirely free from 
internal currents. Further, itis to be granted that all gradations 
may be traced between a banded structure and distinct crystals. In 
a broad sense, however, the rule stated in these terms may be of 
some value. 
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