THE DAWN OF LIFE 1 67 



transmitted light, a yellowish or brownish colour, 

 and in a specimen decalcified with an acid 

 an opaque white appearance. In some of the 

 larger threads of serpentine, as already stated, this 

 mineral forms a thin outer cylinder with a core of 

 calcite or dolomite within ; but this appearance is 

 not common. Here and there, especially in the 

 lower layers, a portion of a tube is filled with 

 the harder mineral pyroxene, which is in some 

 respects similar to serpentine, except that it con- 

 tains lime as well as magnesia, and is destitute of 

 water as an ingredient. The finer tubuli into 

 which the canals ramify are most usually filled 

 with dolomite or magnesian limestone, which has 

 a glossy appearance and higher lustre than the 

 surrounding calcite, and so may be distinguished 

 even in a transparent slice ; but these fine dolo- 

 mite threads are best seen when the surface of a 

 slice is treated with a dilute acid in the cold, in 

 which circumstances the calcite is dissolved, while 

 the dolomite remains as tufts of delicate cylindrical 

 hairs, presenting often a very beautiful appearance 

 under the microscope. Thus, as in many other 

 fossils, what are supposed to have been tubes and 

 tubuli are found not empty, but filled with matter 



