
Parr Il. $ ii] ROCK-FORMING MINERALS. 81 
- amygdaloidal sheets are the chief repositories of zeolites, while the 
firm, compact, columnar beds are comparatively free from these 
alteration products.’ | 
Kaolin, pure clay or hydrous silicate of alumina (silica 46:3, 
alumina 39°8, water 13:9) resulting from the alteration of potash 
and soda felspars exposed to atmospheric influences, is white, but 
~ may be variously coloured by impurities. Ordinary clay is similarly 
formed, but contains iron, lime, and other ingredients, among which 
the débris of the undecomposed constituents of the original rock 
_ forms usually a marked proportion. 
Tale, usually in foliated, inelastic scales, scaly aggregates or 
rosettes with very perfect basal cleavage; white or greenish with 
pearly lustre. H. 1—1‘5. Gr. 2°69—2°80. Composition—silica 
63°5, magnesia 31°7, water 4°8; not soluble in acids. Occurs as an 
essential constituent of tale-schist, and as an alteration product re- 
placing mica, hornblende, augite, olivine, diallage, and other minerals 
in crystalline rocks. Under the microscope appears in small scales, 
_ which, cut transverse to basal cleavage, show ragged edges and an 
internal fibrous structure, the fibres not being parallel as in muscovite; 
is not pleochroic; polarization colours, bright yellow and red. 
Chlorite includes several varieties or species occurring in small 
etreen hexagonal tables or scaly vermicular or earthy aggregates. 
H. 1—1°5. Gr. 2°78—2:95. Composition variable—silica 25—28, 
alumina 19—23, ferrous oxide 15—29, magnesia 13—25, water 
9—12. Under the microscope appears markedly radiated in thin 
plates or spherulites with internal confused radiating fibrous 
structure. An essential ingredient of chlorite-schist. Occurs 
abundantly as an alteration product (of hornblende, &c.) in fine 
filaments, incrustations, and layers in many crystalline rocks. 
Serpentine, not crystallized, or at least only fibrous, granular, 
and compact, breaking with a dull conchoidal sometimes smooth 
_ splintery fracture. H.3—4. Gr.2°5--3°7. Dirty-greenish, yellowish 
_ reddish or brownish colours; often streaked and veined. Consists of 
a hydrous magnesian silicate, viz., silica 43°48, magnesia 43°48, water 
13:04, with a little ferrous silicate. Under the microscope it pre- 
sents in very thin slices a pale leek-green or bluish-green base, show- 
ing aggregate polarization. ‘Through this base runs a network of dark 
opaque threads and veinings. Sometimes among these veinings, or 
through the network of green serpentinous matter in the base, the 
form of original olivine crystals may be traced. There can be 
little doubt that serpentine is, in most cases at least, a product of 
the alteration of pre-existing minerals, and especially of olivine. It 
occurs in nests, grains, threads, and veins in rocks which once con- 
tained olivine,” (p. 77), also massive as a rock, in which it has replaced 
olivine, enstatite or some other magnesian bisilicate. This massive 
condition is described at p. 151. 
1 See Sullivan in Jukes’ Manual of Geology, 3rd edit. p. 85. 
2 See Tschermak, Wien. Akad. lvi. 1867. 
