VARIETIES OF AGATE, ETC. 135 



successive supplies of silica arrived. This orifice has been called 

 the point of infiltration. If the silica did enter in solution in this 

 way, it is not easy to understand how it could arrange itself 

 around the walls of the cavity, and on the roof as well as on the 

 floor in layers of equal thickness. To overcome this difficulty, 

 Dr. Lange, of Idar, has suggested that after gelatinous silica, or 

 silica in solution, had been deposited on the floor of the cavity, 

 an accession of temperature caused the water to boil, and that the 

 pressure of the steam forced the jelly in all directions, and that 

 when the tension became too great it pierced an exit. Hence he 

 calls these apertures irruptive canals. 



Another theory to account for the equal deposition of the layers 

 is that, as the saturated solution entered by the aperture, a current 

 would be produced between the solutions outside and inside to 

 establish an equilibrium, and the current sometimes passed out at 

 another aperture opposite, only partially coating the cavity, but 

 frequently rising above and passing round the roof and out at the 

 same opening by which it entered. 



Dr. Rensch has tried to imitate the formation of an agate by 

 introducing into an irregularly formed cavity a thin cream of 

 plaster of Paris, shaking it round and pouring it out, when a layer 

 is left lining the walls of the hollow. This he repeated with 

 various coloured creams, forming successive layers until the cavity 

 was filled. A section of the nodule cut through presented the 

 appearance of a banded agate. He supposes that the cavities in 

 the amygdaloidal rocks were alternately filled and emptied by the 

 action of intermittent thermal springs. 



In many agates a succession of varying bands continues to 

 near the centre, which frequently is filled up with compact silica, 

 chalcedony, or jasper ; but very often the centre is occupied by 

 quartz crystals. Sometimes the crystals meet and blend so as to 

 form a compact crystalline or semi-crystallised centre ; at other 

 times the crystals from each side just touch, showing a division 

 line between them. Sometimes they do not meet, but leave a 

 hollow studded with pyramidal crystals of quartz, rock crystal, or 

 amethyst ; these hollow nodules are called geodes. The shell is, 

 of course, a material of about a quarter of an inch thick, and 

 covered inwardly with globular concretions. These, again, are 



