490 OVERLYING ROCKS. 



cumstance when it is found in the syenites, as it 

 is in Sky : on account of the resemblance which 

 they then put on to the granites ; a circumstance 

 already noticed under the head of granite. To 

 distinguish these two cases, a rigid geological 

 examination is necessary ; but the important 

 views which this resemblance involves, is a sub- 

 ject for geological discussion. Mica occasionally 

 occurs in the claystones also, examples of which 

 are found in the schistose isles of the Argy shire 

 coast. 



Quartz also, is found in the greenstones, as 

 well as in the syenites ; being intimately mixed 

 with the other ingredients, as it is in granite, 

 and, adding much to the difficulty of distin- 

 guishing them from some varieties of that rock, 

 without having recourse to their geological con- 

 nections. Where it is accompanied by mica, as 

 in the rock of Sky above mentioned, the resem- 

 blance between the two is perfect. Olivin is 

 most frequently found in the situation of an 

 imbedded mineral, like the zeolites ; but it also 

 occasionally forms an integrant part of the mixed 

 structure. Very rarely, mesotype, prehnite, stil- 



