Crystalline Limestone, 361 



At present, we hardly know of a more remarkable example 

 of the first mode of occm*rence, than that presented by the 

 Wenlock limestone of the West of England. This member 

 of the upper division of the Silurian system, is both under- 

 laid and over-laid by shale, whose nature is well indicated by 

 the name of mudstone, which has been assigned to it. The 

 Wenlock formation consists partly of irregular beds of impure 

 argillaceous limestone, and partly of " ball- stones," as they are 

 termed ; and it is of the latter that we have here to speak. 

 Murchison, in his " Silurian System" (vol. i. chap. 17-)> de- 

 scribes them as " concretionary lumps,'' generally varying in 

 diameter from a few inches to several feet, but occasionally at- 

 taining an immense size, consisting of a pure crystalline lime- 

 stone, which is sometimes quite a white marble, full of petri- 

 factions, and surrounded by beds of shale and impure lime- 

 stone. The strata generally terminate suddenly, when they 

 encounter these masses ; but in some places the stratification 

 seems gradually to cease at the place of meeting, so that the 

 concretions imperceptibly unite with the strata. The latter 

 are, for the most part, much contorted, where they surround 

 the concretions ; and Murchison's observation on this point is 

 remarkable. He says, that these contortions have, for the 

 most part, been formed during that process of solidification or 

 crystallization, which gave rise to the concretions of the lime- 

 stone, and cannot be considered as resulting from the disloca- 

 tions by which this tract has been agitated, for such con- 

 cretions enter intimately into the structure of the Wenlock 

 limestone. 



Concretions, which must be regarded as quite analogous to 

 the true ball-stones, occur also in the miderlying shale. These 

 are of a spheroidal form, consist of argillaceous limestone, but 

 also sometimes of pure crystalline limestone, and have in part 

 a structure similar to the well-known *' cone in cone" of the 

 lias formation. Such masses have been found containing 

 crystals of quartz, crystals of calcareous spar, flakes of anthra- 

 cite, and organic remains. In the Wenlock limestone itself, 

 veins of calcareous spar and copper pyrites frequently occur. 

 Murchison, when he first examined them, thought that they 

 might communicate downwards with fissures, connected with 



