QUARTZ: ITS VARIETIES AND FORMATION. 185 



would consist of chalcedony or jasper in the inner portions, and have 

 distinctly crystallized exteriors. There is another class of agates 

 composed of external bands of chalcedony or jasper, stalactitically 

 deposited in a cavity which may either have a hollow center, or 

 one filled up with crystals of quartz. There appear, however, to 

 be intermediate varieties in which concretionary or stalactitic forma- 

 tions have been combined with, or interrupted by, other modes of 

 growth. 



Some of the most curious and beautiful abates are those containing 

 dendritic crystallizations ; in these we see, in the more or less trans- 

 parent chalcedony, which in these agates is not banded, wonderful 

 mossy or confervoid-like growths, often very closely resembling vege- 

 table forms. The valuable stones from Mocha contain ferruginous 

 brown or black inclosures, while some of the dendritic agates from 

 India are filled with a bright-green network of what appear to be fila- 

 ments of conferva?. These dendritic forms in the moss-abates are 

 mostly the oxides of iron or manganese ; or in the green Indian peb- 

 bles, delessite or chlorite. The question of their origin is a difficult 

 one. In some agates the dendrites may have resulted from a segrega- 

 tion of the oxides of the metals from the colloid or partially crystallized 

 silica ; in other cases they may be the effect of subsequent infiltra- 

 tions ; or, again, the quartz may have been consolidated around pre- 

 viously existing crystallizations. With regard to infiltration by these 

 oxides, it is well known that even the most compact-looking chalcedony 

 is permeable, as it is possible by steeping it in solutions of the aniline 

 or other dyes to impart the most brilliant tints to agates, the dye un- 

 doubtedly gaining access to the interior of the specimen through the 

 interspaces of its minutely crystalline structure. 



In a large group of agates, of which beautiful specimens come 

 from India, an appearance of banded formation is seen, which, upon 

 microscopic examination, resolves itself into an infinite number of red 

 or brown spots, regularly arranged in bands or concentric groups : 

 these spots appear to be segregations of oxide of iron. I have not 

 seen a specimen of this species of agate cut sufficiently thin to show 

 whether the arrangement of these minute spots is dependent upon a 

 banded structure in the chalcedony itself, or whether it is independent 

 and the result of molecular force which has determined the arrange- 

 ment in question. It may here be noticed that a vast number of the 

 Indian agates come from the neighborhood of the Gulf of Cambay. 

 Near Turkeysar there are agate conglomerates intercalated between 

 beds of laterite which belong to the Eocene period. These conglom- 

 erates we may suppose to have been derived from the denudation of 

 the earlier igneous rocks which abound in the same district. Uruguay, 

 in South America, also produces a large number of remarkably fine 

 banded agates. Sometimes well-formed quartz-crystals will be found 

 inclosing other substances, which, in some instances, have been caught 



