part 2] GEOLOGY OF THE TBEFJUW FY KITES DEPOSIT. 113 



the result of weathering. Un weathered, they seem to be com- 

 posed sometimes of magnetite and sometimes of iron silicate. 

 Sulphuretted-hydrogen solutions, aided by heat and pressure, will 

 readily combine with oxides of iron 1 and less easily with iron 

 silicate to form sulphides of iron ; whether the outcome is pyrrho- 

 tite or pyrites depends on local conditions, and any silica present 

 would crystallize out as quartz. We have seen that quartz-veins 

 occur in the ore at Trefriw, and that the interstitial material visible 

 on a polished surface also appears to be quartz. 



C. Van Hise 2 points out that the oxidation of pyrites to 

 magnetite is accompanied by a shrinkage of 37'48 per cent, of the 

 volume. We may expect that the converse change from magnetite 

 to pyrites will be accompanied by a corresponding expansion, and 

 this may help to account for the curious structure locally called 

 ' zebra,' which is found between the ore and the shale, or, in the 

 absence of the ore, may occur next to the intrusive rock. ' Zebra' 

 consists of hardened shale full of ramifying pipe-like masses 

 which present the appearance of worm-burrows. ' Zebra ' is 

 clearly a passage-bed from the ore to the shale, and it may be 

 2 or 3 feet thick. Viewed under the microscope it is seen to 

 consist of dark areas, the mass of the shale containing fairly 

 sharply defined clear areas representing the pipe-like bodies in 

 section. In the clear areas pyrites occurs in well defined though 

 not very numerous cubes ; but in the dark areas the sediment 

 seems to have been largely replaced by pyrites. The ' zebra ' 

 sometimes shows a pisolitic structure, an important confirmation 

 of the theory that the pyrites is altered pisolitic iron-ore. 



The absence of the ore in places is important, and can be 

 explained, if we accept the above theory of its origin, by the 

 local absence of pisolitic iron-ore. The pisolitic bed is known, at 

 various places in North Wales, to have a lenticular character with 

 gaps between the lenticles. 



Mr. Morris informs me that, occasionally, the pyrites in the 

 intrusive rock is greatly concentrated near the ore, with the result 

 that the ore-body is at that place thickened by as much as 2 feet. 



The age of the pyrites is most probably Bala, for it is clearly 

 formed, on any theory, by the igneous intrusion and the age of 

 this is most probably Bala. Dr. A. Harker 3 says that there is 

 conclusive evidence that all the diabases associated with the Bala 

 strata of Eastern Carnarvonshire, were injected during the Bala 

 period itself. The heat given out by the intrusion was evidently 

 not great, or its metamorphic effects would have been much more 

 marked than is the case : consequently any effects produced by 

 heated waters proceeding from it would date from a time soon 



1 F. W. Clarke. ' Data of Geochemistry ' 3rd edit. Bull. U.S. Geol. Surv. 

 No. 616 (1916) p. 333. 



2 ' A Treatise on Metamorphism ' Monograph U.S. Geol. Surv. vol. xlvii 

 (1904) p. 406. 



3 ' The Bala Volcanic Series of Caernarvonshire & Associated Rocks ' 

 Cambridge, 1889, p. 76. 



