
110 - GEOGNOSY. [Boox II. 
will find these questions discussed in later pages, and will probably © 
recognise a distinct advantage in this unavoidable reference to them 
in connection with the rocks by which they are suggested. 
§ VI.—A Description of the more Important Rocks of the Earth's Crust. 
Full details regarding the composition, microscopic structure, and 
other characters of rocks must be sought in such general treatises and - 
special memoirs as those already cited (pp. 86, 94). The purposes of 
the present text-book will be served by a succinct account of the more 
common or important rocks which enter into the composition of the 
crust of the earth. | 
A. CRYSTALLINE (INCLUDING VITREOUS). 
1. Stratified: 
This division consists mainly of chemical deposits, but includes 
also some which, originally formed of organic calcareous débris, 
have acquired a crystalline structure. The rocks included in it 
occur as laminz and beds usually intercalated among clastic forma- 
tions, such as sandstone and shale. Sometimes they attain a thick- 
ness of many thousand feet, with hardly any interstratification of 
mechanically derived sediment. They are being formed abundantly 
at the present time by mineral springs and on the floors of inland seas ; 
while on the bottom of lakes and of the main ocean calcareous organic 
accumulations are in progress which will doubtless eventually acquire 
a thoroughly crystalline structure like that of many limestones. 
Ice.—So large an area of the earth’s surface is covered with ice, 
that this substance deserves notice among geological formations. 
Ice is commonly and conveniently classified in two divisions, 
snow-ice and water-ice, according as it results from the com- 
pression and alternate melting and freezing of fallen snow, or from 
the freezing of the surface or bottom of sheets of water. 
Snow-ice is of two kinds. Ist, Fallen snow on mountain slopes 
above the snow-line gradually assumes a granular structure. The 
little crystalline needles and stars of ice are melted and frozen into 
rounded granules, which form a more or less compact mass known 
in Switzerland as Névé or Firn. 2nd, When the granular névé 
slowly slides down into the valleys, it acquires a more compact 
crystalline structure and becomes glacier-ice. The structure and 
movements of glaciers are described in Book III. Part ii. Glacier- 
ice in small fragments is white or colourless, and often shows innumer- 
able fine bubbles of air, sometimes also fine particles of mud. In 
larger masses it has a blue or green-blue tint, and displays a veined 
structure consisting of parallel vertical veinings of white ice full of 
air-bubbles, and of blue clear ice without air-bubbles. |Snow-ice 
is formed above the snow-line, but may descend in glaciers far 
below it. It covers large areas of the more lofty mountains of the globe, 
