[ 358 J 



XXXV.— SUPPLEMENTAEY EEMAEKS ON THE PEEVIOUS 

 PAPEE ON OLDHAMIA. By PEOFESSOE W. J. 

 SOLLAS, LL.D., D.Sc. (With Plate VIII.) 



[Eead, December 15, 1886.] 



The interesting discussion which the observations on Oldhamia, 

 made at the last meeting of this Society, have elicited has induced 

 me to offer the following supplementary remarks : — 



1. The phyllades in which Oldhamia occurs consist of laminae of 

 mica (sericite), clastic grains of quartz, and chlorite. 



2. The boundaries of the quartz grains are usually concealed in 

 the usual thin slices of the rock by the chlorite : on treatment with 

 hydrochloric acid, which removes the chlorite, they are rendered 

 plainly visible. 



3. The surface of the laminae of the phyllades in which Oldhamia 

 does not occur are smooth and even, and in transverse section the 

 constituent minerals are found to be arranged in planes parallel to 

 one another, and to the cleavage laminae. 



4.^0ldhamia presents itself on the surface of the laminae when 

 it is^present as rounded discontinuous ridges, which are without 

 definite boundaries, and have the appearance of fine wrinkles. 



5.°, When the phyllade is fractured obliquely to the cleavage- 

 laminae, the Oldhamia markings are found to extend through the 

 rock, as fine ridges or wrinkles, which mark the surface of oblique 

 fracture in a similar manner to those of the cleavage face. 



6. Transverse sections of such phyllades are wrinkled, conform- 

 ably to the Oldhamia ridges, in minute undulating folds; the 

 sericite scales lie with their faces in the surface of these folds, i.e. 

 they are tangentally arranged. The width of the folds, measured 

 from crest to crest, is about 0*4 to OS mm. ; the Oldhamia ridges 

 have the same width. 



7. The folds traverse several successive laminae of the phyllade 

 for a distance reaching and exceeding 2*0 mm. in length. 



8. A series of such folds traversing several laminae* regarded 

 axially, has the appearance of a narrow band of different texture 

 and colour to the rest of the rock : this is due to the reflection 

 of light from similarly orientated flakes of mica. 



