94 THE MICROSCOPE. June, 



ground down to the proper degree of thinness and need not be 

 removed from the slide. 



A true marble is a crystalline rock, rendered so by the action 

 of heat, moisture and pressure, which has changed its texture 

 from that of a common sedimentary limestone to that of a crys- 

 talline rock in which all traces of animal life have been com- 

 pletely obliterated. 



Of the sedimentary limestones there seem to be two kinds, 

 one halving the fossil remains well preserved and another in 

 which no signs of life can be discovered. In this case it is not 

 to be inferred that the limestone was not of animal origin. There 

 are good reasons for believing that the minute forms of life which 

 would, under other conditions, have made up the rock in a well 

 preserved state, have been dissolved out or disintegrated by water 

 highl}^ charged with carbonic acid. This is especially true of 

 the calcareous shells of foraminiferse, which minute animals live 

 iri a stratum of water near the surface, and, as they die, fall in 

 showers to the bottom, to form, under favorable conditions, beds 

 of chalk or limestone full of well-preserved delicate shells. 



But if the water through which they fall is deep, and under 

 pressure of that depth highly charged with carbonic acid, they 

 would be dissolved before reaching the bottom ; or having 

 reached the bottom, they may in process of time become com- 

 pletely disintegrated, losing their characteristic forms. Prof. 

 W. C. Williamson has given an example confirming this view. 

 In a slab of marble containing a large nautaloid shell there 

 were, in the innermost chamber, foraminifene preserved in the 

 most exquisite perfection, while outside of the shell, where the 

 ooze must have been identical with that in the inside, there was 

 a more or less com])lete disintegration of the foraminiferous 

 shells. Those in the thick nautiloid shell had been protected 

 by it. 



Again, of the limestones having the fossils more or less per- 

 fectly preserved, there are two kinds ; one with the interspaces 

 filled with finely comminuted particles of other fossils, and the 

 other with them filled not with a rnud-like matter, but with 

 clear, glass-like calcareous spar. This was doubtless deposited 

 from water holding calcareous matter in solution, but whose 

 solvent power was diminishing, perhaps by passing from greater 



