THE DIFFERENT KINDS OF ROCKS. 3 



It will thus be seen that ophites are not ordinary rocks. 



In some of the silo-carbacid ophites the presence of a mineral 

 carbonate is feebly expressed ; but in others it is decidedly more 

 pronounced. So abundantly calcareous are some in Northern 

 Italy that they might be taken for carrarite, or dolomite. To 

 the ordinary tourist the cathedral of Milan is built. of " white 

 marble ; " nevertheless this material is so streaked with ser- 

 pentine as to require its being regarded as an ophi-calcite. On 

 the other hand, in Connemara there is a remarkable snow- 

 coloured rock, worked for a while under the belief of its being a 

 "white marble/' that is essentially an aggregation of the anhy- 

 drous mineral silicate, malacolite, and holding little more than 

 a fraction of diffused calcite. It is through such a rock as this 

 that the passage is open, not only into the ophicalcites, but into 

 the purely siliceous metamorphics. 



