THE CONSTITUENTS OF A SEABEACH 57 



in the crag in which they were deposited as pebbles, 

 broken, washed, and rolled ages ago from the chalk. 

 The iron is in a high state of oxidation, and stains not 

 only flint pebbles but the sands of the Red Crag and 

 later gravels a bright orange-red, or sometimes a less 

 ruddy yellow. The iron comes originally from very 

 ancient igneous rocks in which it is black and usually 

 combined with silica. The chalk flints are always, owing, 

 it seems, to minute quantities of carbon, quite black in 

 the mass, but thin, translucent splinters have a yellowish- 

 brown tint. The flints are free from iron stain when 

 taken direct from the chalk. The commonest pebble 

 next to flint is milky quartz, or opaque white quartz. 

 This is derived from some far northern source, where 

 there are igneous rocks traversed by veins of this 

 substance (perhaps Norway). Quartz, like flint, is pure 

 silica, the oxide of the element silicon. It appears in 

 another form as rock-crystal, and also as chalcedony 

 and agate. Opal also is pure silica, but differs from 

 quartz and its varieties in being non-crystalline or 

 amorphous, and in being less hard and of less specific 

 gravity than quartz. Opal is soluble in alkaline water 

 containing free carbonic acid, such as are many natural 

 waters and the sea ! But quartz is not so. The siliceous 

 " spicules " and skeletons of many microscopic animals 

 and plants are " opal." The gem known as " opal " is 

 a variety owing its beauty to minute fissures in its sub- 

 stance which break up light into the prismatic colours. 



A great deal rarer than the milky quartz, but well 

 known on the East Coast on account of their beauty, 

 and often sought for to be cut and polished, are the 

 small rolled bits or pebbles of chalcedony or agate, 

 which have been bedded before their appearance on the 

 beach in some of the pre-glacial or post-glacial gravels, 



