On the Tertiary Iron Ore Measures. 155 



The iron ore measures give as a typical section : — 



Ft. In. 



The floor of the lithomarge usually is an irregular surface of 

 dolerite, which in places is corroded into deep holes ; dolerite is 

 also found in decomposing masses and boulders in the bottom 

 beds of the lithomarge (see Plate XVII.) In some places the 

 decomposed dolerite passes insensibly into the lithomarge, 

 but in o-eneral it is darker than the surroundinof lithomarge, 

 and being vesicular, presents a marked contrast to it. The 

 bottom beds of the lithomarge are generally of a light lavender 

 colour containing numerous white spots of bauxite, which 

 vary in size from a pin head to that of a pea; the upper 

 beds are brown or blackish. It is a brittle and splinty rock, and 

 flies from the pick like flint, but is never hard enough to resist 

 being easily cut with a knife, occasionally it is altered by a dyke 

 into an extremely tough rock. When exposed to the weather it 

 becomes soft and breaks into cubes, which eventually weather 

 into spheroidal forms. Interstratified with the lithomarge are 

 inliers of aluminous iron ore, usually as beds varying from a few 

 inches to several feet in thickness, but often only as lenticular 

 masses. Sometimes small partings of a silicious lignite are 

 found in this aluminous ore, also lumps and thin seams of lignite, 

 but I have never observed lignite in the associated lithomarge, 

 though it often occurs in the pisolitic ore seam, as will be de- 

 scribed hereafter. 



There are interesting circumstances in connexion with the 

 lithomarge when it comes in contact with peat water, which may 

 be here mentioned. When peat water passes over an exposure 

 of lithomarge, oxide of manganese frequently accumulates in the 

 cavities and interstices, one irregular fissure was filled with pyro- 

 lusite (of 70 per cent. Mn. 0.) in places three inches thick, but on an 

 average not more than one inch in depth, the walls came together, 

 and cut out the seam. The manganese would appear to be in 

 solution in the peaty water, and to be deposited by some pro- 



