140 Reports & Proceedings — Edinburgh Geological Society, 



The clay occurs at tlie top of ;t]ie Millstone Grit Series, which in 

 North Ayrshire consists mainly of a suite of basic lava-flows, with 

 occasional interbedderl fireclays, or of ganister-like sandstones. 

 The lava series varies from 5 to 70 fathoms in thickness, and usually 

 rests on another fireclay, " the Monkcastle Clay," which is not 

 bauxitic. 



The upper members of the lava series are often much decomposed, 

 and often j)ass almost insensibly upward either into beds of poor 

 ironstone or of bauxitic clay. 



The clay varies in colour from almost white, through shades of 

 grey, j)ink, yellow, or brown, to jet black ; in hardness from that 

 of a compact limestone to a faky blaes ; and in texture from a 

 compact, practically non-porous rock, with a conch oidal fracture ^ 

 to an almost normal high-grade fireclay ; but as yet it has never 

 been met with in anything approaching to a plastic condition. 



Id some cases it is seen to be a distinct and separate deposit, but 

 in others it appears to represent a metasomatic replacement of a 

 pre-existing rock. Many of the detrital samples show a beautiful 

 development of oolitic structure, which in the main appears to be 

 due to the rolling about of small fragments (sometimes even of 

 broken oolites) on the bottom of a shallow pool of water, Avhich was 

 capable of depositing aluminous material. In a few cases a some- 

 what similar structure has been developed by concretionary action. 



The alumina content varies from 26 per cent to 52 per cent ; and 

 in varieties containing more than 39 per cent (i.e. the amount of 

 AI0O3 in kaolin), practically all the silica is in the combined state 

 as that material. The mineralogical nature of any excess alumina 

 which might be present has not as yet been determined. 



In many ways the bauxitic clay is similar to bauxite, but difiers 

 from it in that the alumina is mainly in a combined form and not 

 free. There can be little doubt, however, that the two materials 

 have similar origins, and it appears likely that the close of the Mill- 

 stone Grit period in North Ayrshire took place under damp tropical 

 conditions ; with the result that the land surface of the lava-flows 

 was subjected to a peculiar type of decomposition known as 

 bauxitization or lateritization, which in this case seems to have 

 been accompanied by reducing conditions, brought about by the 

 decay of large quantities of vegetable matter, producing such gases 

 as carbon dioxide and marsh gas, and thus causing the iron in the 

 minerals present in the basalts to be reduced to a soluble ferrous 

 condition, and thus removable by percolating waters. The alumina 

 also seems to have gone into solution, and from the absence of any 

 form of free silica in the deposit it appears possible that the two 

 " were combined, forming a soluble hydrous silicate of alumina, 

 which was capable of depositing kaolin and free alumina. 



