1873.] XT/CAS — ORIGIN OF CLAY-IEOHSTONE. 367 



to strengthen the present case, as where the iron was obviously 

 discharged into open waters it was generally distributed, and not in 

 local beds. 



I believe, then, that every bed of Clay-ironstone marks a terrestrial 

 horizon as much as every coal-bed does, or as every bed of sediment 

 which bears the print of raindrops. 



Since the above paper was written I have had the opportunity of 

 witnessing, on a small scale, the operation which I suppose to have 

 taken place on a large one. Between Bedcar and Saltburn the 

 shore is guarded by a line of cliffs formed of red Boulder-clay ; and 

 between the foot of these and low-water mark there is a broad 

 extent of smooth sand. Several small streams cut through the clay, 

 and at certain times, not being strong enough to cut out a channel 

 through newly laid sand, they sink through it, leaving a film of red 

 mud (peroxide of iron), together with various vegetables brought 

 down by them, and considerable quantities of sea-weed. Some of 

 this mud I took up in a bottle. In a few days, being closely corked, 

 it turned to a dark blue black, and effervesced with nitric acid. Here, 

 then, we have an absolute case of the formation of carbonate of 

 iron under the circumstances named in this paper. 



A paper by Mr. James Geikie has also lately come into my hands, in 

 which the following passages occur. "The numerous coal-seams point 

 to a recurrence of a land surface, and the gas-coals and ironstones 



to the former existence of numerous wide lakes and lagoons 



Gas-coals are not unfrequently found to pass into common coals, or 

 into black shales, and sometimes into Black-Band ironstones. And 

 this arises from the mode in which these seams were accumulated. 

 Gas-coal has certainly been deposited in water. It contains fresh- 

 water or braclcish-ivater fossils, and may be traced, as just stated, 

 until it is found to pass into a black argillaceous shale. If we con- 

 ceive of a more or less wide expanse of fresh water surrounded by 

 broad stretches of densely wooded flat grounds, we shall have the 

 conditions under which the common coals, gas-coals, and ironstones 

 of the limestone series were most probably formed. Over the bed 

 of the lake would gather a slimy vegetable mud, which in some 

 places might be highly impregnated with ferruginous matter. 

 Here and there this slime or black vegetable mud would pass into 

 a dark clay or silt at points where streams entered the lake ; while 

 all along the shores, wherever the water shallowed, luxuriant growths 

 of marsh-plants would cluster upon the muddy bottom. Beyond 

 this thick marshy growth, again, there would be the drier land 

 covered with enormous trees and a dense undergrowth of ferns. 

 The plants that grew upon the land would give rise to common coal, 

 the marshy vegetation to splint coal, and the dark vegetable silt to 

 gas-coal, oil-shale, and black-hand ironstone; and all these seams 

 would anastomose or pass into each other at certain points. And 

 thus the same mineral scam may be alternately a common, splint, 

 or gas-coal, an oil-shale, or a black-band ironstone, according as 

 the physical conditions varied at the time of its formation. With 



2e2 



