4 
a 
, 
2 2 eee Pe 
—— = 7 
<-> : : 
= . 
\e . 
= 
7 
Spe. 
‘Parr Il. § vi] CRYSTALLINE ROCKS—STRATIFIED. 113 
_ water from which the hanging lime-icicles are derived drips to the 
floor, and on further evaporation there gives rise to the crust-like 
deposit known as stalagmite. Mr. Sorby has shown that in the 
ealeareous deposits from fresh water there is a constant tendency 
towards the production of calcite crystals with the principal axis 
perpendicular to the surface of deposit. Where that surface is 
curved, there is a radiation or divergence of the fibre-like crystals. 
This is well seen in sections of stalactites and of some calcareous 
tufas (Fig. 100). 
Oolite.—A granular limestone, in which the grains are more or 
less perfectly spherical, giving the aspect of fish-roe. Hach grain 
-eonsists of successive concentric coats of carbonate of lime formed 
round some minute grain of sand or other foreign body which was 
_ kept in motion, so that all sides could in turn become encrusted. 
Oolitic grains of this character are now forming in the springs of 
Carlsbad (Sprudelstein); but they may no doubt also be produced 

Fie. 15.—Mocroscoric SrrucrurEe oF Oo.rric LIMESTONE, AFTER SoRBY. 
Maanirrep 30 DIAMETERS. 
where gentle currents in lakes or partially enclosed areas of the sea 
keep grains of sand or fragments of shells drifting along in water, 
which is so charged with lime as to be ready to deposit it upon 
any suitable surface. Where the individual grains of an oolitic 
limestone are as large as peas, the rock is called a pisolite. 
Marble (granular limestone).—A crystalline-granular aggregate 
composed of crystalline calcite granules of remarkably uniform 
size, each of which has its own independent twin lamelle (often 
giving interference colours) and cleavage lines. This characteristic 
structure is well displayed when a thin slice of ordinary statuary 
marble is placed under the microscope (Fig. 16). Typical marble 
is white, but also yellow, grey, blue and red; or streaked and 
mottled. Its granular structure gives it a resemblance to loaf- 
sugar, whence the term “saccharoid” applied to it. Fine silvery 
scales of mica or tale may often be noticed even in the purest 
marble. Some crystalline limestones associated with gneiss and schist 
I 
