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XXXIV.— ON A SPECIMEN OF SLATE FEOM BEAY-HEAD, 

 TEAVEESED BY THE STEUCTUEE KNOWN AS 

 OLDHAMIA BADIATA. By PEOFESSOE W. J. 

 SOLLAS, LL.D., D.Sc. 



[Eead, November 17, 1886.] 



The structure known as Oldhamia radiata commonly presents itself 

 in the form of discontinuous thread-like ridges radiating from a 

 common centre, and lying in the planes of cleavage (coincident 

 with the planes of original bedding) of a slate. A hand specimen 

 in the collection of Trinity College, Dublin, cut transversely to the 

 cleavage planes, shows, however, that the Oldhamia structure is not 

 merely superficial, but extends across the cleavage planes into the 

 substance of the rock. 



In the hope of throwing further light upon this problematical 

 structure, thin slices for microscopic examination were cut from this 

 specimen, both parallel and transverse to its planes of cleavage. 

 On placing these under the microscope all trace of the Oldhamia 

 structure appeared to have vanished. An examination with the 

 unaided eye showed, however, that it was still there, presenting 

 itself as narrow, undulating, and branching bands of a lighter 

 colour than the surrounding matrix: its appearance, however, 

 varied in an extraordinary manner according to the direction in 

 which it was viewed. Looked at obliquely in a strong light, the 

 thread-like bands are brilliantly illuminated, and appear faintly 

 coloured with spectral tints ; looked at directly, the bands become 

 fainter, and are less clearly distinguishable from the matrix. In 

 certain positions the slice taken at right angles to the bedding has 

 an appearance somewhat suggestive of shot-silk, and from the 

 planes of cleavage, markings which somewhat remotely resemble 

 in form the dendritic markings of Sutton stones extend into the 

 surrounding matrix. These appearances, taken as a whole, suggest 

 the presence of some mineral possessing high reflection or refrac- 

 tion arranged in more or less parallel planes. 



My friend, Mr. Teall, to whom I submitted the prepared slides 



2B2 



