Sollas — On a Specimen of Slate with Oldhamia. 357 



that they represent the alteration of ilmenite or titaniferous mag- 

 netite. A few scales of hematite also occur in the slides." 



To this I may add that — (1) some fragments of quartz are pre- 

 sent in the slate, which I think do undoubtedly show traces of a 

 clastic origin ; and (2) that a minute crack traversing the cleavage 

 planes is shown in one of the slices ; it is lined by chlorite on 

 both sides, and filled with quartz containing minute scales of 

 sericite, and numerous air-cavities, both spherical and crystal- 

 shaped. Hematite is also present on the quartz ; this shows that 

 the formation of chlorite and sericite, and quartz, continued to 

 take place even after the cleavage of the slate. 



Examining the slices in the light of this description, one finds 

 that the lighter-coloured bands, which correspond to the Oldhamia- 

 structure, owe their distinction from the surrounding matrix to 

 the presence of an excess of sericite scales ; and that the curious 

 shot- silk appearances are produced by the local deflections of these 

 scales from parallelism with the cleavage planes, into directions 

 tangential to curves, which are probably transverse sections of those 

 long ridges which, when seen on the exposed surface of a cleavage 

 plane, are recognized as the usual form of Oldhamia ; and it would 

 appear possible that these ridges are wrinklings of the cleavage 

 planes produced during the shearing which led to their formation. 



In addition to these corrugations, a structure resembling false 

 bedding, on a small scale, is visible on some parts of the slices. 



