

SKY. GEOLOGY. SILICEOUS SCHIST. 357 



siliceous schist, is fully supplied by the appearances on 

 the western shore of the same promontory ; and as the 

 examples that occur there in situ belong to the upper beds 

 of the series where they are in contact with the super- 

 incumbent trap, it is probable that the fragments on the 

 eastern shore have been detached from the same parts 

 of the deposit. While the connexion between the sili- 

 ceous schist and the chert is here found perfect, the 

 two substances alternating as the limestone and the shale 

 do in the beds immediately succeeding, we can also 

 trace among the different examples a perfect series of 

 transition from the natural to the indurated state. This 

 is perhaps most striking in the shale, which passes by 

 imperceptible degrees into the most perfect Lydian stone. 

 In the case of the chert there is less regularity, but 

 the difference is easily explained by attending to the 

 composition of the original limestone beds. Where these 

 have been tolerably pure, they seem incapable of under- 

 going any change but that of crystallization ; where the 

 other earths have been in excess, they are converted 

 into cherts of various degrees of hardness, in which the 

 calcareous ingredient can no longer be detected by the 

 ordinary methods; the several earths having combined, 

 as in the case of pottery, into a substance not unaptly 

 to be compared to it and requiring methods of analysis 

 equally powerful. In all the specimens of the shale and 

 Lydian stone, shells are occasionally found. Mr. Pennant 

 mentions ammonitae : I have only observed minute bi- 

 valves, their shells being very thin, but the forms so nearly 

 obliterated that it is impossible to conjecture even their 

 affinities. They are never abundant, but I may remark 

 that they are rare in this part of the series, even among 

 the unaltered strata. 



Although in some instances the actual contact of the 

 trap with these indurated strata is here to be seen, it 

 is not always found, apparently in consequence of the 

 degradation of the rocks ; but one very perfect and 



