VARIETIES OF AGATE, ETC. 137 



outside the tube, until the cavity is filled or the growth stopped 

 by the cessation of the supply of silica. Sometimes the original 

 tube remains hollow, but generally it is filled up, leaving a dark 

 spot in the centre, with circles surrounding, showing the marks of 

 intermittent growth, as in the case of the eye agate and other 

 circular forms. 



It has been suggested that some of the forms of fortification, 

 and other irregularly formed agates have been determined by 

 crystals of calc-spar having been formed in the cavities, and after- 

 wards dissolved, and their places filled up with colloid silica. But 

 in many fortification agates, the lines appear to follow the outline 

 of the cavity. 



Professors Woodward, Raskin, and others suppose the lines 

 and markings of many agates procured from the drift are the 

 result of decomposition since the agates were set free from their 

 native matrix and embedded in the drift, as is known to be the 

 case with many banded and variously coloured flints. 



Locality and Geological Position. — -The river Achate in Sicily 

 (how called the Drillo) is said by Theophrastus about 300 B.C. 

 (the earliest Greek writer on stones whose works are still extant) 

 to have given its name to the agate. The agate, he says, is an 

 elegant stone ; it has its name from the river Achate in Sicily, and 

 is sold at a great price. Pliny, in the first century of the Christian 

 era, informs us that the Achate was a stone formerly held in high 

 estimation, but in his day was of little or no value ; that it was 

 first found in Sicily, but has since been discovered in many other 

 places, including Cyprus, Phrygia, Egypt, and India. 



Prof Forbes (in Oriental Memoirs) says the best agates are 

 found in peculiar strata thirty feet under the surface of the earth 

 in a small tract among the Rajpipla Hills, on the banks of the 

 Nerbudda. They are not met with in any other part of Guzerat, 

 and are generally cut and polished in Cambay. 



Agates are now procured from various parts of the globe : 

 Africa, North and South America (especially Brazil), Australia, 

 Russia, China, India, Germany, and Scotland. They are found 

 plentifully in the Amygdaloidal trap-rocks of Scotland, notably at 

 Kinnoul hill in Perthshire, at Burne Anne near Galston in Ayr- 

 shire, near Montrose in Forfarshire, on the beach, in the sands, in 



