28 



INTRODUCTION. 



sufficient to absolutely destroy any organic remains present in the 

 rock. Thus, in the dolomites of the Guelph formation (Silurian) of 

 Canada, the presence of fossils can commonly be recognised in thin 

 sections, though these have always undergone more or less secondary 

 change, and have usually been replaced by calcite or by peroxide of 

 iron, or are represented simply by vacant spaces in the rock. On 

 the other hand, in many dolomites (fig. 10) secondary crystallisation 

 has been as complete as in statuary marble, and any organic remains 

 which may have existed in the rock to begin with have been totally 

 obliterated. 



With regard to the origin of magnesian limestones, it is sufficient 

 here to say that those in which magnesia is present in comparatively 

 small quantity probably owe their peculiarities to the conditions 

 under which they were originally deposited ; and there are even true 

 dolomites in which there is ground for thinking that the rock was 



Fig. 10.— Thin section of dolomite, from 

 Sweden, enlarged ten times. The rock is 

 an aggregate of comparatively large crystals, 

 which exhibit the characteristic cleavage- 

 lines of carbonate of lime. (Original.) 



Fig. ii. — Section of oolitic limestone, Car- 

 boniferous, Kershope Foot, enlarged thirty 

 times. Some of the spheroids have no defin- 

 ite boundaries, and consist simply of diffuse 

 radiating crystallisations. (Original.) 



magnesian from the first. In other cases, again, it cannot be 

 doubted that the rock was originally a normal limestone, and that 

 " dolomitisation " was the result of secondary changes affecting the 

 rock subsequent to its original formation. 



(3.) Mi?iute Structure. — The most important of the ordinary struc- 

 tural peculiarities of limestones is what is known as the "oolitic" 

 structure. The subject of the oolitic structure of calcareous rocks is 

 one of great complexity, and the morphological differences which 

 exist between different oolites are very numerous and highly remark- 



