438 SECONDARY LliMESTONE. 



but, in other cases, they are formed of concentric 

 crusts. 



In the second variety, the spheroidal structure 

 is on a larger scale, and the spherules, which are 

 simple, interfere with each other so as often to 

 present considerable irregularity. On weather- 

 ing, as the intermediate soft parts disappear, they 

 become very visible; protruding in the same 

 botryoidal forms which are hereafter mentioned 

 as also occurring in the siliceous schists. 



The last variety so exactly resembles that 

 already described as sometimes occurring" in the 

 sandstones, that it is only necessary to repeat the 

 remark there made, by observing, that the sphe- 

 roids are, in this case, imbedded and sparingly 

 dispersed in the general mass of an ordinary stra- 

 tum ; that they extend to the diameter of two, or 

 even three feet, are commonly very much flat- 

 tened, and are sometimes associated in pairs by 

 a cylindrical stem. 



Before quitting the subject of structure, it 

 must be observed, that these limestones are fre- 

 quently found to have been fractured and re-united 

 by the same materials. In this way they present 



