86 



THE POPULAR SCIENCE MONTHLY. 



Fig. 3. Arborized Agate ; that is, agate in- 

 closing a dendrite deep in its mass. (Speci- 

 men from the Museum of Natural History ; 

 halt' the natural size.) 



green, or gray, with which the gypsum is cut up, are darkened 

 with dendrites of various dimensions and sometimes very elegant. 

 These dendrites are likewise found in limestones, chalk, building- 

 stones, lithographic stones, and compact marbles ; in sandstones, 

 granite, and various other crystalline rocks. They are not always 



black ; some are the color of 

 rust ; some are metallic, and 

 consist of common pyrites be- 

 tween sheets of slate, or copper, 

 or native silver, or gold. Final- 

 ly, besides superficial dendrites, 

 deep ones are known, which are 

 developed across the mass of the 

 stones. The best-known speci- 

 mens of this kind are those 

 which make appropriate the 

 special designation of arborized 

 agate (Fig. 3). 



This name, like that of den- 

 drites, shows that a vegetable 

 origin was at first attributed 

 to these accidents. Sometimes 

 fancy went further ; and Fig. 4 represents, from Mylius, whom 

 we have already quoted, the figure of a dendrite in which the 

 author saw a landscape a plain traversed by a river and bordered 

 by a chain of wooded hills, and pierced with caves. It is easy to 

 discover that dendrites have none of the characteristics of the 

 vegetable ramifications with which we are at first inclined to com- 

 pare them, and, when we study them under a sufficient magnifying 

 power, the crystalline 

 structure of most of 

 them appears distinct- 

 ly. This is especially 

 the case with the black 

 dendrites, which are 

 most, abundant, and is 

 shown in the originals 

 < f Figs. 2 and 4, which I 

 have particularly stud- 

 ied, and have been able 

 to produce artificially. 

 It is evident that 

 these dendrites, which consist of a hydrated oxide of manganese 

 the acerdesis of mineralogists are the result of a precipitating 

 action exercised by calcareous rocks on water containing traces 

 of metallic salts. Hence we might expect to obtain an imitation 



Fig. 4. 



-Schist, exhibiting dendrites, in which the repre- 

 sentation of a landscape may be imagined. 



