104 -  GEOGNOSY AND GEOLOGY. | 
in chemical combination. Lime predominates. Marl forms masses which 
are either slaty or indefinitely shelly. The lamination of the rock is very 
decided, causing it to fall to pieces readily. | 
Lime Marl. Nature has drawn no lines between the different kinds of 
marl. The species can be determined only approximately by the prevailing 
component. An artificial limit has been set between them. According to 
this, lime marl, of which there are the varieties, marly limestone, marly lime 
slate, and marly earth, must contain over seventy-five per cent. of carbonate 
of lime. When it contains between seventy-five and fifty per cent., it is 
called clay marl. This occurs in not inconsiderable masses; and in an 
agricultural point of view is as important as the marls in general. The 
color and hardness vary considerably, the former being greyish, yellowish, 
reddish, greenish, and white. The grey and black colors are due to 
bituminous particles, the green to chlorite. Clay marl is not decomposed, 
but crumbles to pieces readily ; water penetrating between the laminz and 
into the pores by capillarity, experiences an expansion by heat or freezing, 
which splits the rock into fragments. 
Fetid Marl. Marl often contains so much bitumen as not only to be 
colored brown or black, but to emit a strong odor when struck. The outer 
coating is frequently white, the effect of evaporation or abstraction of the 
bituminous particles. F’etid mar] rock is distinguished from fetid marl slate 
and fetid marl earth. The three varieties are only defined by their external 
appearance. The fetid marl slate, also called bituminous marl slate, is in 
many places entirely impregnated with copper ores; on which account it is 
in many places mined and worked for copper, notwithstanding that it 
generally contains only three per cent. of the metal. The bitumen which 
penetrates the rock is often separated in a pure state, and singularly enough, 
vrincipally in places where there are organic remains; so that the 
supposition that the bitumen depends upon such remains, and is nothing else 
than a product of decomposition of organic matter, seems to be not entirely 
without foundation. 
Section 4. Fetid Limestone 
Embraces those limestones that are so transformed by coaly or bituminous 
substances, as to possess a dark color, and to diffuse a bituminous odor 
when struck. 
Fetid Lime, or bituminous limestone. It is divided into fetid lime, fetid 
clay, oolitic fetid lime, breccious, and porous or cellular. The more 
bitumen the rock contains, the darker are its colors, which are generally 
grey, brown, or brownish-black. Those portions exposed to the air are 
generally lighter, often entirely white, the inside remaining dark. This is 
caused by the passing off of the bitumen leaving the rock somewhat porous. 
The bitumen is often separated as asphaltum. The rock is often penetrated 
by other foreign matters. Thus there is frequently a fetid quartz 
corresponding with the rock crystal in pure limestone. 
Anthraconite, or carbonate of lime with a considerable amount of 
carbonaceous matter. This occurs compact and scaly granular. Threads 
of white limestone, or brownspar, often run through it. 
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