148 



He should confine himself, however, to the first and last of them. 

 The first, Beekite, was interesting as displaying the mammillated 

 'structure with concentric and semi-concentric rings, and as replacing 

 to a great extent the limestones on which it was formed. In process 

 of time the li-mestone decayed, and a hollow siliceous crust remained. 

 The Beekites were found in great numbers in the Triassic conglomerate 

 of Devonshire. Professor Rupert Jones (Proceedings of the Geologists' 

 Association, Vol. iv., No. 7) traced to a similar cause the origin of the 

 potato-stones found in the Triassic breccia of Somersetshire. They 

 had a siHceous geode, the interior of which was lined with quartz 

 crystals ; these last being formed from the siliceous solution which 

 filled the cavity after the limestone was removed. Soinc of the most 

 beautiful masses of chalcedony were those rounded pebbles found on 

 the Sussex coast, and wrongly termed moss-agates. For, as Professor 

 Jones remarked, they were not agates, and contained no moss. The 

 problems connected with them for which he wanted solutions were— 

 Whence do they come ? Are they ever found in the chalk ? How 

 are they found, in geodes or hollows of flints, or are they independent 

 concretions .'' Is the large quantity of oxide of iron associated with 

 them an accident or a necessity of their existence ? 



" Agates are either solid nodules of chalcedony, usually in more or 

 less concentric layers, or hollow nodules of the same (geodes) with 

 crystallised quartz coating the inner surface (druses) or filling the 

 middle. The central quartz is often amethystine." Briefly, an agate 

 was formed in this wise. Take the case of a highly \esicular lava. 

 The cavities were foi"med when it was in a fluid or plastic state by the 

 steam and gases which were developed within it. Most so-called 

 igneous rocks contained such cavities to a greater or less extent. 

 Irregular cavities were also formed as the rock contracted in cooling. 

 Liquids holding silica in solution found a resting place in these hollows 

 and deposited the silica. By and bye the rock decayed and a banded 

 mass of chalcedony and quartz crystal was left behind, being more 

 durable than the rock. In illustration of this process he had brought 

 some exquisite little agates still i7i situ, from the basalt of the great 

 Teesdale dyke. 



Among the highly interesting problems connected with these 

 was the cause of their banded structure. These bands offered unequal 

 resistance to the solvent powers of hydrofluoric acid and to alkaline 



