320 A. GEIKIE ON THE SUPPOSED 



a resemblance to mineral veins. Traced along the strike, they are 

 found to be lenticular walls imbedded in and shading off into shales 

 and fine tuffs. 



The material composing these vertical bands occurs likewise in 

 nodules or concretions varying from the size of a pin's to that of a 

 man's head or larger. These are particularly abundant in a bed of 

 grey shale, the laminae of which bend round them, as if nodules of 

 some kind had lain there when the sediment was being deposited. 

 A gradation in size and in development seems to be traceable among 

 these included masses. In some cases they are represented by 

 cavities lined with limonite. In proportion as they increase in size 

 they grow more flinty in texture, until they assume the same pale 

 milk-white translucent character found among the bedded masses. 

 One of the concretions which M. Renard has kindly analyzed for me 

 has the subjoined composition. 



Analysis of Concretion (Adinole) Nun's Chapel, St. David's. 

 By M. Eenard. 



Si o 78-62 



Al 2 3 13-67 



Fe 2 3 1-22 



Mn trace. 



Mg trace. 



Ca 0-30 



K 2 0-26 



Na 2 5-80 



Loss 0-63 



100-50 



The layer of shale next one of the thick bands, or round a concre- 

 tion, is frequently dull, dark green in colour, and compact in texture, 

 so that on a wet face of rock the contrast between it and the adjoin- 

 ing pale indurated substance is well marked. 



There is still a third form in which the same cherty material 

 occurs. It may be seen ramifying through the shales in strings and 

 winding veins, that strike across the stratification of the surrounding 

 beds. It is there welded, as it were, into the shale, the laminae of 

 which pass into it. Faint lines may even be detected here and there 

 passing across the veins in the line of the laminee of the shale, on 

 either side, as if they were a survival of these laminae. In the veins, 

 as in the bands and concretions, the portions next the shale are 

 more granular than the central parts, which as before, become flinty, 

 milk-white and translucent. 



Dr. Hicks and Mr. Davies have compared these compact flinty 

 aggregates with the halleflintas of Scandinavia. They are undoubtedly 

 products of metamorphism ; and their chemical composition as well as 

 minute structure probably varies from place to place. The rock, 

 analyzed by M. E-enard corresponds both in composition and in 



