A more detailed description will now be given of the mineral phosphates principally employed in the manufacture, and of those waste products from other branches of industry which are now made to contribute towards the production of artificial manures.
Coprolites.—The name " coprolites " is given to a large class of mineral phosphates, existing as nodules and fossils, in strata of various geological ages, and scattered widely over the globe. They have been supposed to be fossil animal excreta, but it ia at least doubtful whether that is the true origin of all coprolites.
The most valuable beds of the mineral in this country are in the Upper Greenaand formation, lying chiefly in Cambridgeshire, and merging into Buckinghamshire. These are known as "Cam bridge " coprolites, and formed at one time almost the only mineral phosphate employed in this country. They are of a greenish•grey tint, and are washed from a stratum not exceeding 1 ft. in depth. Hitherto, their standard of tribasic phosphate has been very constant ; but deterioration is now often visible. They contain more carbonate of lime than the other coprolitea, while the iron present is almost entirely as sulphide and silicate. The principal ingredients vary as follows :— Tribasic phosphate of lime, 54-60 per cent. ; carbonate of lime, 11r184 ; fluoride of lime, 1I-43 ; oxide of iron and alumina, insoluble ailiceoua matter, 6-8g.
Suffolk coprolites are another variety, raised in that and some adjoining counties, from beds adjacent to the London clay, in the Tertiary formation. That decomposed organic remains have at least been the origin of their phosphoric acid, can scarcely be questioned ; but it is doubtful whether these coprolitea are of the same character as the "Cambridge," and whether they have not rather been calcareous pebbles, metamorphosed by the action of phosphoric acid. They were the first coprolites used in this country, and monopolized the market till better varieties were discovered ; but they are much inferior to Cambridge, in containing less phosphoric acid, and much more oxide of iron and alumina. Alone, they are not sufficiently good for making a superphosphate which shall yield at least 25 per cent. of soluble phosphate ; but they may be used with richer phosphates,
such as Cambridge coprolites, in the proportion of 3 parts of the former to 1 of the latter. They much resemble Cambridge coprolites in shape, but are very hard, with a smooth surface, and brownish-ferruginous tint. Their tribasic phosphate of lime averages about 52I-614 per cent. ; carbonate of lime, 10-171; fluoride of lime, ; iron and alumina, 4g-l0i ; and insoluble matters, n-124.
At Potton, in Bedfordshire, coprolites m quarried from the Lower Greensand formation ; on the whole, they are inferior in quality to the preceding. The larger nodules equal " Cambridge" in proportion of phosphoric acid, but contain more oxide of iron ; the lesser nodules are of very poor quality. The colour is reddish. Their approximate composition ia :—Tribasic phosphate of lime, 30-544 per cent.; carbonate of lime, 5-8; fluoride of lime, 4; iron and alumina, 8-20; insoluble matters, 15-25. The nodule bcd ia intercalated in a deposit of coarse ferruginous sand, and is exceedingly variable in thickness. The sand extends to the north-east of Saudy Heath, by Everton, (lamlingay, and Caxton, but the phosphatic bed has not been discovered in this direction. The extension of both the sandy and the phosphatic beds in a south-weaterly direction is deter mined by the river Ivel. Tho beds reappear at Ampthill, where they are worked for the coprolites, and may also be traced in the neighbourhood of Woburn and Leighton.
Wicken coprolites are an inferior kind from Cambridgeshire, generally resembling the Suffolk variety, and composed approximately of :—Tribasic phosphate of lime, 36 per cent. ; carbonate of lime, 10 ; fluoride of lime, 2 ; iron and alumina, 12 ; insoluble matters, 28. The phosphatic nodules are embedded in a sandy matrix, and are of two colours, light and dark : the former resemble those of Potton ; the latter are characterized by a smooth exterior, and a smaller percentage of phosphate. The workings have been abandoned for some time.