18 Mr. W. M. Holmes on the Microscopical 



particles showed traces of organic origin. The second portion 

 has a similar constitution, but the particles are somewhat smaller. 

 The third portion consists principally of grains of glauconite, and, 

 as this mineral is very abundant throughout the greensand for- 

 mation, I propose to discuss its constitution rather fully. 



The mineral receives its name from its green colour, and it 

 occurs in small grains or in masses formed by the union of 

 several grains. It occupies a prominent position both in space 

 and time, its occurrence extending from the Cambrian period to 

 the most recent post-tertiary layers. Concerning the manner of 

 its formation, there has been much controversy, without any very 

 definite solution being arrived at. 



Messrs. Murray and Renard, in their Report on the Deep Sea 

 Deposits collected by the 'Challenger,' have paid great attention 

 to this subject, and I have received great assistance from this 

 volume. They state that the grains of glauconite are generally 

 homogeneous, and are almost always accompanied either by 

 quartz, orthoclase, white mica, plagioclase, hornblende, magne- 

 tite, garnet, epidote, tourmaline, zircon, or fragments of ancient 

 rocks such as gneiss, mica-schist, chloritic rocks, granite, dia- 

 base, &c., and in modern deposits by organic matter, often of a 

 vegetable nature. 



Dr. Hinde called the attention of this Club, in 1886, to the 

 occurrence of sponge spicules containing this mineral in the fire- 

 stone rock of Grodstoue, and two years ago I read a short note on 

 glauconite casts from the same locality. 



Firestone appears to differ from hearthstone by containing less 

 lime in its composition, and by its much greater hardness when 

 dry. 



Glauconite, both recent and fossil, consists of a hydrous 

 silicate of potash and ferric oxide, with variable quantities of 

 alumina, ferrous oxide, magnesia, and often lime. It is formed 

 in the cavities of foraminifera, sponge spicules, and other 

 organisms, but as to the conditions which determine its forma- 

 tion little is known. The shells and other organisms in which 

 it is formed are sometimes broken by the growth of the glauco- 

 nite, and the isolated cast then becomes a nucleus upon which 

 fresh additions may be deposited. Although generally found 

 associated with foraminifera and other organisms, the mere 

 presence of these is not in itself sufficient to cause its formation, 

 as it is absent in many coral muds and sands, and in many 

 pteropod and globigerina oozes. It is never formed in a free 

 state in the mud. Glauconite is one of the restricted number of 

 silicates formed at the present day on the sea-bed, not universally 

 distributed, but limited to deposits now forming in relatively 

 shallow depths in more or less close proximity to continental 

 land, and especially along those high and bold coasts that are 



