618 KANSAS CITY REVIEW OF SCIENCE. 
ing a part of the rocks of this formation. ‘These limestones are in the form of 
marble, but all the rocks of the Laurentian system are highly metamorphic. 
These limestone beds are of great thickness and of vast geographical extent. 
Three bands of these, varying in thickness from 60 to 1500 feet, have been 
traced in Canada for more than 100 miles, and they are doubtless of much greater 
extent. 
It is almost universally conceded that limestones, in all ages of the world, 
are of organic origin, having been built up from the stony forms of polyps and 
the shells of mollusks and crustaceans. That this is the manner in which the 
limestones of later ages have been built up does not admit of question, and we 
know of no good reason for assigning any other origin to those of this very early 
period. If, therefore, these extensive beds of limestone were produced by animal 
agency, it would necessarily demand the contemporaneous existence of enormous 
quantities of vegetation. Another fact that seems to demand a similar cause for 
its explanation is the existence of large quantities of carbonic acid in combination 
with the lime in these rocks. There is strong reason to believe that all carbonic 
acid originally existed in the air, as we find it still so existing to the extent of 
about .o4 per cent. It is through vegetable agency that this gas is abstracted 
from the air and fixed in a solid form. It is held in this form till by the decay 
or combustion of the organic substance it is liberated again to enter into the 
atmosphere in a gaseous state. It was doubtless through vegetable agency that 
this substance was withdrawn from the air that it might be laid up in the stones. 
Admitting the correctness of this view, it would indicate a very abundant vegeta- 
tion during the time these rocks were forming. That these limestones were of 
organic origin is further rendered probable by the fact that they are associated 
with hydrous silicates; especially serpentine and loganite, which may arise from 
the facility with which silica combines with bases in the presence of organic 
matters, or from the abundance of soluble silica in the hard parts of diatoms 
which probably formed the chief food of those animals that build their own 
skeletons of carbonate of lime. ‘These facts create an almost irresistible presump- 
tion that the limestones of this period were of organic origin, which, if true, proves 
beyond question the prevalence of plants at, and probably long prior to, the time 
of their deposition. 
A second indication of the existence of vegetation during the Laurentian 
period, is found in the existence of abundance of carbon besides that which 
exists in combination with lime in the limestones. This is found in the form of 
graphite or plumbago. This is simply one stage of coal, having probably existed 
in the form of ordinary coal at an earlier period of its existence. Coal exists in 
all degrees of carbonization, from the poorest quality of lignite to the pure sub- 
stance in the form of graphite. The various stages of progress through which 
this process is carried on are indicated by lignite, bituminous and anthracite coals 
and graphite. ‘The stage reached seems to depend on the extent of metamorphic 
agencies to which it has been exposed. In those regions, as in Northern Kansas, 
