410 Br. Mac Culloch on the Gealogy of 



having been subsequently filled up by red or white sparry matter, a 

 beautiful variety of marble is produced, which is now wrought for 

 the purposes of ornament. Its cavities also occasionally contain 

 large and perfect crystals of that form of calcareous spar called 

 equiaxe. Its surface is in most parts covered with a loose calcareous 

 tufo, which in some places being solidified by an infiltration of cal- 

 careous matter, forms beds of solid breccia. It contains grains of 

 sand dispersed throughout, and for this reason, commonly gives fire 

 with steel, a circumstance of very frequent occurrence in all stratified 

 limestones, whether referred to the rocks called flcet%^ or transitio7t^ 

 and generally most conspicuous in those which contain animal 

 remains in abundance. Such are the circumstances of chief im- 

 portance in the character and position of this limestone. 



I shall now, perhaps, be expected to assign a place to this rock in 

 the usual division oi primary^ transition^ and^^/z, distinctions which 

 1 am inclined to think are more easily made in the closet than in 

 the field. In the present state of geological science, it would appear 

 a safer practice in this case, as in many others, to describe that which 

 actually exists, without the use of hypothetical terms, which only 

 serve to perplex the observer, and to mislead the student, who either 

 boldly pronounces on the character which suits his particular creed, 

 or modestly supposes himself incapable of sound observation, because 

 he is unable to see that which is not visible. If the quartz rock of 

 this tract be a primitive rock, then the limestone is necessarily a 

 primitive rock also, notwithstanding its bituminous nature, and 

 accurately stratified structure, since it alternates with it. But, if this 

 rock is not a primitive, but a transition rock, then the limestone also 

 must be removed from the first class of rocks to the second. 



In examining its right to a place among the transition rocks, we 

 must compare its characters wuth those which have been assigned as 



