130 THE ORIGIN AND HISTORY OF 



best known are the flints of the chalk, chert of the green- sand and 

 limestone, and the clay ironstone nodules of the coal measures. 

 The Egyptian jaspers of the Nile Valley and some flints from the 

 chalk are frequently called Agates on account of the concentric 

 and other peculiar markings which they exhibit. 



Agates are hard, siliceous nodules of fine texture, transparent 

 to translucent, formed by the infiltration of siliceous waters into 

 cavities in various kinds of volcanic and igneous rocks ; and con- 

 sist, to a large extent, of successive alternate concentric layers of 

 various kinds of chalcedony, jasper, and opal ; and quartz in the 

 form of rock crystal and amethyst, and these sometimes are 

 covered with chalcedony when the cavity has only been partially 

 filled, as is the case in many geodes (hollow nodules). Speci- 

 mens may be seen with an outer circle of amethyst and amethyst 

 crystals on one side of the cavity, and quartz crystals lined with 

 chalcedony on the other. Many geodes are lined with amethyst 

 or quartz crystals only. When fresh supplies of siliceous waters 

 have been continued, the central cavity has been filled by the 

 quartz crystals on each side, meeting in the centre. 



Agates receive different names in accordance with the arrange- 

 ments of bands, stripes, spots, and clouds of various colours, as 

 red, green, blue, yellow, white, brown, and black, with which they 

 are inwardly variegated. Those which have stripes or bands of 

 various colours arranged in straight parallel lines are called ribbon 

 agates on account of their resemblance to ribbon. The term, 

 fortification agate, is applied when the parallel bands assume a 

 zigzag angular arrangement, somewhat in the form of a fort. 



Eye Agate consists of a number of concentric rings of various 

 colours surrounding a dark central spot, and somewhat resembles 

 a human eye ; sometimes the circles appear to be sufliciently 

 round to have been drawn by a compass. I have a fair specimen, 

 with a reddish-brown iris, surrounded by four alternate circles of 

 bluish white, and black or very dark brown chalcedony, surrounded 

 by a band of crystalline quartz. 



Moss Agate is a translucent chalcedony, which appears to 

 include embedded moss, ferns, seaweed, small vegetable filaments, 

 and other arborescent forms. It is sometimes called tree agate, 

 and some of the varieties are called Mocha-stone, either from 



