THE ORIGIN AND HISTORY OF AGATES, ETC. 129 



taking part of the weight of the grain of corn, and backing across 

 ahead of its companion until it had got it to a safe place. After 

 helping one it returned to meet another, and continued this 

 apparently voluntary task as long as this systematic robbery 

 lasted." — Scientific American. 



^bc ©rigin an^ Ibistor^ of IDacietice of Hoate, 

 jflint, au^ other Siliceous IRobules. 



By C. D. Hardcastle, Leeds. 



A GATES have been defined as siliceous concretions or con- 

 IX. cretionary nodules, formed in cavities in the trap and 

 kindred rocks. They can scarcely be said, however, to be 

 concretions in the ordinarily accepted meaning of that term. 



A concretion — from the Latin con and cresco, to grow together — 

 is an aggregation of mineral matter of a different kind from the 

 matrix in which it is embedded, formed round a central point or 

 line, attracted generally by some organic matter, as a sponge, fern, 

 leaf, or shell, which frequently remains enclosed, but is sometimes 

 nearly, if not altogether, obliterated. These may be illustrated by 

 sponge in flint, Echinus in flint, etc., fern and leaf in Dudley iron- 

 stone, Ammonite in lias, and Productus in coal measure sandstone. 



Agates — as a general rule, at all events — are formed by infil- 

 tration of silica into cavities in rocks by consecutive layers, laid 

 on the walls of the cavities in the first instance, and continuing 

 towards the centre, which is filled up last ; or, perhaps, in some 

 cases, the cavity is filled up almost at once with colloid silica, 

 much in the way that a mould is fiUed with metal in casting. 

 There is no organism enclosed, as in concretions On this account 

 it has been proposed to call agates secretionary or incretionary 

 nodules. 



Concretions of various kinds are formed in all classes of sedi- 

 mentary strata from the alluvial sands and clays of recent date, to 

 the deposits of paloeozoic age. Nodules of carbonate of lime 

 occur in recent and tertiary clays— jaspers in the sandy clay 

 deposits of the Nile, and menelite in other tertiary deposits. The 



International Journal of Microscopy and Natural Science. 

 Third Series. Vol. VI. l 



