MAKVLANIJ (iKOr.fXilCAL SLIiVKY 55 



Tliu glaiK'oiiitic I'lciiiciits art- sccoiidarv in cliiirnctcr iiinl wcrt- formed 

 in situ. Siiicr iliry ciik'r so lai-;:('ly iiiln I lie rdiniatiuii uf the Eocene 

 greensaiuls. (Iicii- cliaraclci- ami oii-iii will he i-()ii>i(lcrcil .soiiH-wliat more 



fully. 



ORUilX OF CIHEKXSAND. 



Great light has been lln-nwn npnn this siiljjcct hy the results of the 

 deep-sea dredgings which have l)een made in recent years l)y tiie vessels 

 sent out on scientific expeditions under government auspices. The 

 most important of these expeditions was that of the Challenger, sent out 

 by the British Government in the years 1872-76. In the report upon 

 the deep-sea deposits, based upon the dredgings of that expedition. 

 Professors Murray and Eenard, the authors, present the results of their 

 researches as to the character and distribution of greensand, and at the 

 same time propose a theory to account for the chemical changes which 

 have taken place to produce the mineral glaueonite, its chief constituent. 



The gianconite occurs both in existing seas and in geological deposits 

 as minute grains, seldom exceeding 1 millimeter in diameter, althougli 

 these grains may at times become agglomerated into nodules several 

 centimeters in diameter by means of a phospliatic cement. The grains 

 are always more or less rounded, and at times mammillated, with irregu- 

 lar surface outline. The}' are generally black or dark green in color, 

 but become brighter green upon being crushed. The surface of the 

 grain is sometimes covered with fine punctures, while at other times it 

 is smooth and shining. Some of these glaueonite grains are distinct 

 internal casts of foraminifera and of other calcareous shells; but more 

 often they are only indistinct reproductions of the form of the chambers 

 and show no definite connection Avith the organisms in which they orig- 

 inated. In the Eocene deposits the foraminiferal casts are less distinctly 

 seen than in the deposits of recent seas, yet even here they are not im- 

 common. 



It is estimated that giauconitic deposits cover approximately 1,000,000 

 square miles of the sea floor, while they are found at nearly all geolog- 

 ical horizons from the Cambrian up. On the present ocean floor they 

 are limited to those portions adjacent to the coasts, and for the most 

 part along the higher parts of the continental slopes, where land- 



