~ 
92  @EOGNOSY. °° Boo TT.” 
Delessite, in kernels or incrustations, with a finely fibrous or 
delicately scaly internal structure; olive to blackish-green. H. 2— 
9-5. Gr. 2:89. Composition—silica 31:07, alumina 19°47, ferric 
oxide 17:54, ferrous oxide 4°07, magnesia 19-14, lime 0°46, water 
11:55, the iron being sometimes entirely as protoxide. Gives off 
water in matrass and becomes brown; easily decomposed im acids 
with residue of silica. Occurs abundantly as a decomposition product 
of augitic rocks, coating or filling amygdaloidal cavities or narrow 
filamentous veins. 


Glauconite. A soft greenish granular mineral of variable com- _ 
position, found in many stratified formations, particularly amon 
sandstones and limestones, where it envelopes grains of sand, or fills 
and coats foraminifera and other organisms, giving a general green 
tint to the rock. Silica 47-58, alumina 3—10, ferric oxide 0—22, 
ferrous oxide 8-—22, magnesia 0—6, lime O0—2°5, potash 45—9, 
water 5‘5—14-7. It is at present being formed on the sea-floor off 
the coasts of Georgia and South Carolina, where Pourtales found it 
filling the chambers of recent polythalamia. 
CarBonaTes. This family of minerals furnishes only four which 
enter largely into the formation of rocks, viz., Carbonate of Calcium 
in its two forms, Calcite and Aragonite, Carbonate of Magnesium — 
(and Calcium) in Dolomite, and Carbonate of Iron in Siderite. 
Calcite. Rhombohedral, but with great diversity of crystalline 
forms, most frequently in rhombohedra, as in the form called “nail-— 
head spar,” in sealenohedra, as in “ dog-tooth spar,” or in hexagonal 
prisms; also fibrous, granular, or pulverulent; white, but often stained 
with impurities ; lustre vitreous to dull. H. 3. Gr. 2:-6—2'8. Cleavage 
rhombohedral, very perfect, giving angles of 105° 5’ and 74° 5’, 
Composition—calcic carbonate or carbonate of lime, Ca Co,, but 
frequently with some ferrous or manganous oxide, &c., and often 
with enclosures of other minerals. Hffervesces easily with acids. 
Occurs as (1) an original constituent of many aqueous rocks (lime- 
stone, calcareous shale, &c.), either as a result of chemical deposition - 
from water (cale-sinter, stalactites, &c.), or as a secretion by plants 
or animals’; or (2) as a secondary product resulting from weathering 
when it is found filling or lining cavities, or diffused throuch the 
apillary interstices of minerals and rocks. 1t probably never occurs 
as an original ingredient in the massive crystalline rocks, such as 
granite, felsite, and lavas. Under the microscope, calcite is readily 
distinguishable by its intersecting cleavage lines, by a frequent twin 
lamellation (sometimes giving interference colours), strong double 
refraction, weak or inappreciable pleochroism, and characteristic — 
iridescent polarization tints of grey, rose and blue. 
Ir m h > TeEA( i 38S al T j . ye yy ¢ ~ a P * . . 
om the readiness with which water absorbs carbon dioxide, from 
! Mr. Sorby has recently investigated the condition in which the caleareo tt 
of the harder parts of invertebrates exists. He finds that in foraminife a soi 2 
derms, brachiopods, ‘crustacea, and some lamellibranchs and gastero ods it ¢ cae 
calcite; that in nautilus, sepia, most gasteropods, many lamellibraa ee &e, it 
aragonite ; that in not a few cases the two forms occur together, or that the carbor te OPE 16 
is hardened by an admixture of phosphate. Quart. Journ. Geol Soc, 1879 ""Addrens ial 
. Soe, \ » p. 61: 
