4b 
Directions for Geological 
[Feb. 
Some beds which are highly crystalline ;*that these have been formed by the debris, 
or by the particles of older rocks cemented together, must be considered doubtful ; 
they would rather lead to the idea that they may have been formed in the same 
manner as the crystalline rocks, as greenstone, primitive limestone, &c. &c. 
Limestone is a carbonate of lime, that is, composed of carbonic acid and lime; 
besides its external character, described in works on mineralogy, it is most readily 
known from its effervescing with acids, by the disengagement of the carbonic acid 
gas, the lime remaining in solution in the acid. 
According to the position of the beds of limestone in the series which will be af- 
terwards explained, and also according as its structure varies, it receives different 
names. 
Primitive limestone is found alternating with granite gneiss and mica slate. It 
is highly crystalline, being composed of a number of small crystals united together ; 
these, when separated, shew the rliomboidal clearage, as in calcareous spar. Statuary 
marble is the finest variety ; the color is, however, often grey and blueish-grey or 
veined. The primitive beds do not contain shells or organic remains ; all beds which 
contain shells or organic remains, are superior to the primitive beds. Beds of cry- 
stalline limestone, which contain shells, &c. are termed transition limestone. 
Mountain limestone, which forms so great a portion of the English series, (see 
fig- 7.) and on which the great coal formation rests, is a transition limestone. It is 
generally of a dark-grey or brown color, but never yellow ; it contains frequently a 
great deal of bitumen, so as to be black, and cavities in it are frequently found 
filled with mineral pitch or oil ; it contains a great many shells and corals, often 
running through it in seams. 
Lias limestone, also forming a portion of the English series, is a compact lime- 
stone, with a dull fracture, compared to the primitive ; it contains generally a por- 
tion of alumina, is best known from the fossils which it contains, and is sometimes 
called gryphite limestone. It is generally of a dark-grey or blue cokr, but some 
of the seams nearly white. This latter variety has been employed for lithography, 
but not found to answer so well as the Bavarian stones. 
Oolite , which gives name to a large portion of the English rocks, has its name 
from being composed of small rounded grains like eggs, or the roe of a fish ; it is 
a pui’e carbonate of lime, like primitive limestone, but its granular composition is 
quite different from the crystalline appearance of the latter. 
Chalk requires no description ; it is a pure carbonate of lime, but less deuse or 
compact, than the preceding. 
Marl is a carbonate of lime, mixed with argil or other substance. 
Calcareous Tufa is formed by the deposits of springs and rivers containing lime 
in solution, and is therefore a formation now in progress ; it is generally porous 
and of a dead earthy appearance ; it often forms round leaves, twigs of trees, &c. of 
which it retains the impression after their being removed. 
Stalactite is formed on the roofs of caves and fissures, by water containing lime in 
solution, percolating through the roof, and depositing it in the form of icicles; 
when these drops come too quick to part with all their calcareous matter in the roof, 
and form a similar deposit on the ground, they are termed stalagmit. 
Magnesian limestone, called also dolomite , is a carbonate of lime and magnesia, 
containing 20 per cent, of the latter ; it is harder than the carbonate of lime, and 
does not effervesce easily with acids, unless when heated. It does not form so good 
a mortar as pure limestone, but is considered a durable building stone ; it is consi- 
dered injurious as a manure, excepting in small quantity ; the colour is generally 
yellow and aspect sugary, with frequently rhomboids crystallized through it. No 
shells are found in this limestone. It may be observed, that there is an°impercep- 
tible transition of these beds into each other, from primitive limestone to marl; and 
many of the limestone beds are impure, mixed with earthy matter. When it is 
wished to ascertain the quantity of lime in such beds, it may be done by weighing 
the limestone, and a quantity of acid sufficient to dissolve it ; and on weighing it 
again, after it is dissolved, it ought to have lost, if it be pure carbonate of lime, 43 
matter^’ * hence> the excess ot weight above this will give the quantity of earthy 
Gypsum, the sulphate of lime, is generally found nearly pure : from it is made the 
p aster ol Paris, by drawing off the water of crystallization by heat ; the powder is 
• e V° re . absorb water > and on being mixed with it becomes hard, or sets. Gypsum 
i I s ‘oguished from the carbonate by not effervescing with acids, and by being less 
formation of Eigland 1 ^” 68 U “ bedl ' o£ rock 5alt > a,ld “ thu3 found ia the red marl 
1 
\\ 
