266 Field Columbian Museum — 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 in the rate of deposition. The 

 layered structure is the result of trickling 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, it is 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. 



