1 68 THE JOURNAL OF GEOLOGY. 



by the vessels sent out on scientific expeditions by the various 

 governments of Europe and our own. The most important of 

 these expeditions has been that of the Challenger, sent out by 

 the British government in the years 1872-76. In the report 

 upon the Deep Sea Deposits recently published as a result of 

 that expedition, Professors Murray and Renard, the authors, 

 present the latest results upon 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 glauconite which characterizes all greensand deposits. 



A typical greensand is composed of glauconite associated 

 with greater or less amounts of land-derived material. Among 

 the more common minerals thus found are quartz, feldspar, horn- 

 blende, magnetite, augite, zircon, epidote, tourmaline and garnet, 

 together with fragments of the continental rocks, such as gneiss, 

 mica-shist, granite, diabase, etc. A variable amount of calcareous 

 matter derived from the shells of organisms is also present. 



The glauconite occurs in minute grains, seldom exceeding i 

 mm. in diameter, although they may become agglomerated into 

 nodules several centimeters in diameter by means of a phos- 

 phatic cement. The grains are always more or less rounded, 

 and at times mammillated, with irregular surface outline. They 

 are generally black or dark green in color, but become brighter 

 green upon being crushed. The surface of the grains is some- 

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

 smooth and shining. Some of these glauconitic grains are dis- 

 tinct internal casts of foraminifera, and of other calcareous 

 shells, but more often they only reproduce indistinctly the form 

 of the chambers, or show no definite connection with the organ- 

 isms in which they originated. 



It is estimated that greensand deposits cover approximately 

 1,000,000 square miles of the sea floor. They are found limited 

 to those portions adjacent to the coasts, and, for the most part, 

 along the higher parts of the continental slopes where land-de- 

 rived materials are deposited in perceptible, yet small arnounts. 

 The production of glauconite seldom reaches to greater depths 



