MINERALOGY: R. A. DALY 
661 
cates, including very pure albite. The largest feldspars seen were from 
2.5 to 3.0 mm. in length. He suggested that the albite crystals origi- 
nated, as tiny individuals, at the contact of sea-water and the calcareous 
mud at the sea-bottom; and that these grew to present size in the loose 
sediment as deposition went on. Only on such a hypothesis, he held, 
could one explain the automorphic character of the crystals. Since 
''certainly considerable time was necessary for their formation before 
being completely enclosed in the mud," he regards these crystals as prov- 
ing the slowness of the calcareous precipitation. Foullon notes the 
absence of appreciable lime in the feldspar molecule itself. 
Cayeux^ stated that similarly authigenic^ orthoclase is to be found in 
all the named horizons of the Chalk of the Paris basin. The crystals 
are always minute, averaging 0.04 to O.OvS mm. With constant crystal 
forms, they occur singly, never in groups. It is observed that orthoclase 
and glauconite tend to exclude each other in the various limestones. 
Cayeux assumes a community of origin for both minerals, described as 
"secondary." After examining his material, Michel Levy, Lacroix, and 
Termier all agree that the orthoclase must have formed in situ. 
Grandjean^ studied the chalk of Meudon, near Paris, concluding that 
its feldspar (probably microcline, rather than orthoclase) was formed on 
the floor of the Cretaceous sea, ''contemporaneously with the deposition 
of the chalk." The feldspar "grew rapidly; it probably ceased to grow 
after it was once buried." He continued: "it is thoroughly [infiniment] 
probable that feldspar is forming on the present sea-bottom." 
A review of these European studies leads to the following generaliza- 
tions : 
1 . Authigenic alkaline feldspars appear in non-metamorphosed lime- 
stones of Triassic, Jurassic, Cretaceous, and Eocene age and specially 
characterize many horizons in the Alpine Jurassic and Paris-basin 
Cretaceous. 
2. Albite or orthoclase may occur alone or they may be associated in 
the same layer of limestone. Pyrite, bituminous matter, and bipyra- 
midal quartz are not uncommon companions of the feldspars. One 
Jurassic limestone exhibits authigenic mica. 
3. These feldspars in the European limestones typically appear as 
single crystals, with good crystal form and separated by the calcareous 
matrix; their lengths range generally from 0.03 to 0.08 mm. 
4. All the calcareous matrices have the grain and texture of chemical 
precipitates. However, embedded in those originally muddy materials 
are the strongly contrasted fragments of organic origin, shells and 
skeletons. 
