iBiv-^ii.j 403 



from Iceland. The outside of the onyx is generally coated with a 

 thin film of a "green earth" closely resembling chlorophaeite, 

 while between this and the surrounding basalt there is invariably 

 present a thin coating or layer of a dark-brown substance identical 

 in appearance and in many of its properties with hullite. 



Calcite is rarely present and then it always occurs between 

 this outside layer and the onyx, sometimes filling one-third to 

 one-half of the cavity. In one specimen (exhibited) this is clearly 

 shewn, and on approaching the junction of the onyx with the 

 calcite we find perfect little rhombs of the latter detached and 

 embedded in the former, as if they had crystallized therein while 

 the opal was in a nascent state. Under a high power the onyx 

 shews a typical microscopic structure of prismatic and spherical 

 crystallites floating in a ground of isotropic opal. 



A microscopic section of the rock proves it to be a very fine- 

 grained basalt of the basaltic-andesite type, consisting chiefly of 

 augite and a plagioclase felspar, shewing ophitic structure and 

 occasionally fluxional structure. The felspar predominates and 

 is porphyritic in places. Olivine appears to be absent and 

 magnetite is very scarce in a typical section of the rock. The 

 magnetite and other oxides of iron rapidly increase, however, close 

 to and around the cavities containing the onyx. The interstices 

 of the rock are filled with the dark-coloured earth-like hullite. 

 Under the microscope this latter substance varies in colour from 

 light brown (tinged with green) to dark brown. It agrees with 

 hullite in being istotropic in uniform sections and affecting 

 polarised light where radiated. On examining the rock with 

 medium powers (^ inch objective) one cannot help being struck 

 with the resemblance between it and the basaltic-andesite of 

 Carnmoney Neck as seen with low powers (2 inch objective). 

 This rock from Spanish Bay is the type of lava that we imagine 

 must have flowed from a volcanic neck of the Carnmoney type. 



As in the case of the Carnmoney rock it is quite evident that 

 the interstitial "hullite" or brown earth in the Spanish Bay basalt 



E 



