SILURIAN BEDS. IL 
evidence there is, therefore, goes to show that the dolomite is of Lower 
Silurian age. . 
Origin of the dolomite—It is practically agreed upon by geologists that 
dolomite as a rock is nearly always formed by the alteration of calca- 
reous sediments subsequent to their deposition.' The structure of the 
Silurian dolomite at Aspen, as seen under the microscope, is not that of 
a sedimentary deposit, since it is made up of interlocking crystals and 
of quartz which has evidently formed in place without any important 
amount of detrital material. This structure is sufficient evidence that it 
has crystallized through the influence of solutions; and the question pre- 
sents itself as to whether this rock was deposited as such from oceanic 
waters, or whether it is the result of the alteration and recrystallization 
of an original simple sedimentary deposit. The theory of a chemical pre- 
cipitation of carbonate of magnesia requires the assumption of a shallow 
evaporating salt lake or inland sea, in which carbonates of lime and 
magnesia derived from solution of rocks were concentrated and finally 
thrown down. 
Bischof,” however, has shown that owing to the difference in solubility 
between the carbonates of lime and of magnesia the lime carbonate will 
be nearly all deposited from a saturated solution containing both these 
salts before the precipitation of the magnesia carbonate begins; conse- 
quently there would result trom a deposit formed in this way a lower 
layer of nearly pure carbonate of lime and an upper layer of nearly 
pure carbonate: of magnesia. There might be some mingling of the 
carbonates in the zone between the two layers, but no important amount 
of dolomite could be formed in this way. There are no cases known 
where such a process of formation of dolomite is now going on. 
On the other hand, there have been many cases described where dolo- 
mite has been produced along narrow zones which often cut across the 
bedding of the limestone. Such alteration has been described by Hark- 
ness*® in the south of Ireland, where narrow bands of dolomite have been 
produced along joints, and the conclusion is inevitable that here the dolomite 
has been produced by the metamorphosing action of waters which have 
penetrated the limestone along these joints at some period subsequent to the 
' Dana, Manual of Geology, fourth edition, p. 133. 
* Chemical and Physical Geology, Vol. III, p. 170. 
*Robert Harkness, Quart. Jour. Geol. Soc. London, Vol. XV, p. 100. 
