BLUE MARL. 445 



blue marl usually forms the whole thickness of the bed. More 

 eastward, and lower down the country, it sometimes forms the 

 whole of low-lying beds, but more usually only the lower layers of a 

 bed, of which the upper part is yellow. 



Blue marl is generally such as remains " in place," or where the 

 shells were left by the death of the enclosed animals, and the inter- 

 mixed earth is mostly silicious sand ; and therefore (and not because 

 of its colour), this marl is rarely found as rich as 45 per cent., and 

 is still more rarely equal to the yellow clay marls, though generally 

 richer than the yellow sandy marls. 



Blue marl in the bed is always wet, being made so by water 

 slowly oozing from every part, though seldom fast anywhere, or 

 showing springs or veins of running water. The blue colour is not 

 caused by moisture (for some yellow marls are also permanently 

 wet), but by vegetable extract or other dark-coloured organic mat- 

 ter, brought in the percolating water. This inference I have drawn 

 from extensive observation of the natural beds, and also from seve- 

 ral accurate though accidental experiments, of which the first that 

 was observed will be here stated. A small stable yard was covered 

 6 to 10 inches thick with a rich dry yellow marl, for the purpose 

 of retaining by chemical combination the juices of the putrescent 

 manure which was to be thrown there from the stable. After re- 

 maining for this use a year or more, this flooring of marl was dug 

 up and carried out for manure ; when it was found to be changed 

 in colour to a deep and vivid blue, and precisely like the natural 

 colour and appearance of the under-stratum of the same body of 

 marl, which being an open and almost pure mass of pulverized (and 

 water-borne) fragments of shells, was readily penetrated by and 

 always full of water. A general fact confirming this view is that 

 all marls found lying immediately under swampy soils, full of 

 vegetable matter, are blue. And this colouring vegetable matter 

 in marl is not merely intermixed with, but must be held in chemi- 

 cal combination by the calcareous matter; and serves, according to 

 its quantity, in blue marls, as an addition to the fertilizing power 

 of the calcareous matter alone. The particular body of marl above 

 referred to, the under-stratum of which is the most marked or vivid 

 blue ever seen in marl, is at Shellbanks farm, Prince George, and 

 from which I dug and applied a large quantity. The greater part, 

 and all the richest layers, seemed to be of shells broken down to a 

 coarse powder, or of sizes less than fine gravel, through which clear 

 water rose and passed so freely as to forbid digging to the bottom. 

 The small quantity of clay or other earth intermixed with the cal- 

 careous earth of this marl is altogether insufficient to hold so much 

 colouring matter; and moreover, if the colouring matter were not 

 chemically combined with the calcareous, the continued free pas- 

 sage of water must have dissolved and washed off any uncombined 

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