94 



ago of bird-frequented islands in a Tertiaiy Sea. The phosphate or 

 phosphorite from these deposits is known commercially as Bordeaux 

 phosphate — from its port of shipment — and though very irregular in its 

 occurrence, is largely worked and exported. 



Phosphatic deposits like these, however, directly referable to 

 sui'face accumulations by sea birds, are as a rule quite modern. With 

 rare local exceptions any which may have been found in the earlier 

 geological jjsriods have been washed away and lost; the very process of 

 submersion, necessary as a px'eliminary step to the preservation by 

 burial in the strata, causing their dissipation. 



Most of the truly fossil phosphates found in connection with the 

 older rocks have been formed in a quite different manner. To under- 

 stand this we may examine first such modern deposits as the "Mussel 

 Muds" of Prince Edward Island. These are accumulations produced 

 in shallow tidal estuaries where great numbers of molluscs and other 

 marine organisms are going to decay, so rich in phosphates and organic 

 matter as to be of great value locally as a manure. Deposits more or 

 less closely resembling these are found in many parts of the modern 

 sea bottom and along the coast, and where just such deposits have 

 been buried deei)ly, and included in some of the older formations, they 

 produce what are known as " Coprolite beds." This term, however, it 

 must be explained, is in general very loosely applied. It should be 

 restricted to the fossil excrements of various animals, which are 

 ■occasionally found in the rocks, and often in such beds as those just 

 referred to, but seldom even then constitute more than a small part of 

 the phosphatic matter, most of which usually occurs as concretions or 

 nodules. These have resulted from that slow process of drawing 

 together of like particles in the mass, which is usually designated 

 concretionary action, but is not in all cases fully understood. A frag- 

 ment of shell, or bone, or a tooth frequently serves as the nucleus of 

 such a concretion, and when the material is abundant such ccncretions 

 frequently coalesce and form almost continuous layers. The so-called 

 coprolite beds of Cambridgeshire, Bedfordshire and other localities in 

 England, and those of Carolina, in the Southern States, are of this 

 nature. 



The last named deposit dates no farther back than the Tertiaiy, 



