I 'itii Tuyl — New Points on the Origin of Dolomite. 257 



As regards the possibility that some dolomites may be of 

 clastic origin, none has been found which exhibits any signs of 

 clastic structure. But that the original structure might have 

 been obliterated during recrystallization is easily conceivable 

 in rocks of this type. 



Turning now to the dolomites which from their field rela- 

 tions are known to be secondary after limestone we have much 

 more definite data. Indeed in these, by virtue of the fact that 

 the alteration has frequently been halted before it proceeded 

 to completion, we are often able to trace all stages of dolomiti- 

 zation from a limestone showing only incipient alteration to a 

 good dolomite. Thus, it is possible to describe the steps 

 normally passed through during the transformation of a lime- 

 stone to dolomite. 



So far as the testimony of the microscope goes the fine- 

 grained limestones are more susceptible to alteration than the 

 coarser-grained ones, a fact which is in keeping with the laws 

 of chemistry. The evidence also indicates that the alteration 

 may not proceed in exactly the same manner in the two types 

 of limestone. 



The alteration of fine-grained compact limestones seems to 

 be accompanied normally by a notable increase in size of grain. 

 Usually the diameter of the dolomite crystals formed is many 

 times as great as that of the original calcite grains. But in 

 rare cases, such as that of the dense Middle Devonian dolomite 

 referred to above, the original structure and texture seems to 

 be approximately retained. In the dolomitization of such fine- 

 grained limestones the replacement frequently begins at many 

 centers throughout the rock and spreads outwards from these, 

 or if the rock possesses fine stratification the replacement may 

 follow closely these original lines of weakness in the early 

 stages. In those cases where the alteration begins at certain 

 centers and spreads out from these, fucoid-like markings occa- 

 sionally serve as the nuclei as in the case of the Tribes Hill 

 limestone. But as a general rule no organic influence is noted. 

 Normally 7 the limestone is altered uniformly in the process of 

 spreading from the dolomite centers, but it cannot be said that 

 it is altered completely, for the dolomite patches often possess 

 less than twenty per cent of MgC0 3 . Small remnants of lime- 

 stone, however, may occasionally escape alteration and become 

 incorporated in the dolomite patches. The boundary between 

 the limestone and the spreading dolomite area may or may not 

 be abrupt. When abrupt, the rock may assume the appearance 

 of a breccia and the term " pseudo-breccia " may aptly be 

 applied. When the boundary is gradational, on the other 

 hand, rhombohedrons of dolomite, variable in size but usually 

 nearly perfect in their development, are disseminated through 



