402 



[Proc. B.N.F.C.. 



analcite in geodes of small clear glassy crystals, and the other is 

 the onyx which is the subject of this paper. 



About three years ago I traced the specimens of onyx to 

 their source on the cliffs above Spanish Bay, and since then I 

 have collected numerous specimens from this locality. The exact 

 situation, where the onyx is found in situ, is the large scree of 

 broken masses of basalt, half-way between the eastern extremity 

 and the centre of Spanish Bay. The centre of the Bay is well 

 marked by the large dyke which cuts through the basalt and 

 forms a natural pier for small boats. The onyx occurs in the 

 gas-cavities of a fine-grained lava constituting the cliffs above the 

 Bay. The under portion of the rock is greatly weathered at this 

 point, and consequently the overhanging masses of basalt are 

 continually giving way and falling down into the scree below. 

 These large blocks of basalt, when broken up, yield the specimens 

 I am about to describe. 



The cavities in which the onyx occurs are of all shapes, 

 varying from the typical spherical amygdale to mere cracks or 

 interstices, but the most common form of cavity is of an irregular 

 shape with a more or less flattened floor. Independent of the 

 shape of the cavities is the fact that almost without exception they 

 present the typical horizontal structure of onyx-opal as their 

 content, comprising alternate layers of milky-white and clear opal. 

 Very occasionally a cavity is found to be lined with a layer of 

 chalcedony, inside which the onyx is then found, and sometimes 

 leaving a hollow space in the upper portion of the cavity coated 

 with small crystals of quartz. Sometimes the cavities are com- 

 pletely filled with clear opal shewing no sign of onyx structure 

 except when carefully examined in section under the microscope. 

 Again the cavities may be filled with a beautiful variety of milk 

 opal, shewing a reddish-yellow opalescence, or with yellow opal of 

 a honey colour. The milk-opal and honey-opal generally occur 

 in vein-like cavities. Some of the specimens from Spanish Bay 

 strongly resemble similar specimens from the Faroe Islands and 



