LOWER CHALK—MICROGRAPHIC STRUCTURE. 289 
Farther east the amount of the detrital minerals is exceedingly 
small in comparison with the bulk of the Chalk, but quartz grains 
measuring not less than '20 mm. in their longest diameter occur 
in nearly every specimen. In the Sponge-Bed of Hunstanton 
grains measuring 1 mm. are to be found. Neither Dr. Hume nor 
I have observed aggregations of quartz grains similar to those 
described by M. Cayeux from the Upper Turonian of France.* 
For the general report by Mr. J. J. H. Teall, F.R.S., on the 
Detrital Minerals of the Lower Chalk, see p. 311. 
Secondary Minerals.—Glauconite. — On modem sea floors this 
mineral is usually associated “ with the minuter fragments of the 
rocks end minerals of continental lands,” and /‘its greatest deve¬ 
lopment takes place where fine muddy particles commence to 
make up a considerable portion of the deposit.” In the zone of 
Am. mrians it occurs most abundantly in the sandy Chalk Mail 
of Mupe Bay, Avliere it is estimated to form 3' per cent, of the 
rock 10 feet above the glauconitic marl, though at 20 feet 
above the proportion is little more than 0'5 per cent, the 
amount diminishing upwards as the detrital minerals become 
scarcer. 
It continues to occur sparingly throughout this zone, and Was 
found in specimens obtained from the cliff-sections of the south- 
coast and in all counties south-west of Cambridgeshire, the grains 
being small and widely separated in the rock, but to the northward 
they become very scarce, and are finally almost entirely confined 
to the Spcnge-Bed and Inoceramus-Becl of Hunstanton. In 
Lincolnshire and Yorkshire, it becomes still more rare. 
Except in the lowest beds of this zone, and in the Inoceramus- 
Bed of Hunstanton, casts of Foraminifera in glauconite are 
uncommon. At Mupe Bay the larger grains are rough andmam- 
millated and resemble those of the Cliloritic Marl, but most of 
the grains seen in this zone are small and rounded, and vary in 
colour from a dark olive-green to a pale green. 
Their maximum size approximates closely to those of the detrital 
mineral grains. The largest measured was '80 mm., in its longest 
diameter, and occurred in the lowest specimen of the marl from 
Mupe Bay, in which locality the average size was '31 mm. In 
the remainder of this zone the maximum size scarcely ever exceeds 
•55 mm., the majority of the grains measuring from '10 mm. to 
■ 15 mm. 
Marea-site. — Small brassy nodules of this mineral occurred in 
only two or three instances. They were found in the specimens 
from Lydden Spout, near Dover, and from Arlesey in Bedford¬ 
shire, both as rounded irregularly-shaped nodules, lengthy cylinders 
* Contrib. a letude microgr. des. Terr. Sed. p. 336 (1897). 
+*“ Challenger ” Reports, Deep Sea Deposits, p. 313. 
