JUKES-BHOWNE ! MICROSCOPIC STRUCTURE OF ZONES OF CHALK. 387 
less fine-grained material, which encloses a greater or less number of 
larger particles, such as fragments of shell or Foraminifera, Some- 
times the matrix forms the greater portion of the material; sometimes 
the definite organic and inorganic particles are in larger quantity. 
The matrix may be either wholly calcareous or may include some 
very fine argillaceous matter, and occasionally the mass is bound 
together by a cement of crystalline calcite. 
The larger particles may be classified under four heads — Shell 
Fragments ; Perfect Shells ; Sponge Spicules ; Mineral Grains. 
(1) Shell Fragments. These are small angular fragments of the 
calcareous shells and spines of various marine animals. The most 
abundant are pieces of Inoceramus shell, varying in size from what 
may be called a large piece, under the microscope, down to single 
detached and broken prisms. Next to these in abundance, at some 
horizons, are fragments of the plates and spines of Echinid shells. 
Portions of the shells of Terebratula, Rhynchonella, Ostrea, and other 
genera can sometimes be distinguished. Broken Foraminifera are 
also common. 
(2) Perfect Shells. Of these the most universally diffused are 
very small hollow spheres, which have often been mistaken for the 
detached or primordial cells of some kind of Foraminifera. Dr. G. J. 
Hinde, however, informs me that he has no doubt they are perfect 
shells and not detached cells ; they occur also in modern oozes, but 
it is not known to what organism they belong. 
Foraminifera are sometimes common, but are never so abundant 
as to occupy much space in the field of a slide. Both arenaceous and 
porcellanous forms occur. Entomostraca occur in some beds, and 
both they and Foraminifera may have contributed largely to the 
material of the fine calcareous dust of the matrix. 
(3) Sponge Spicules. These are frequently present and are 
sometimes abundant, but are not always in their original condition, 
being often entirely replaced by calcite. 
(4) Mineral Grains. These are partly such as have been formed 
on the sea-floor, and partly mineral fragments derived from terres- 
trial sources. Glauconite is the most common mineral ; it generally 
occurs in grains of rounded outlines, and as it is often seen filling 
r 
