various parts of Scotland, 421 



stone, provided their proportions are not conspicuous, nor their in- 

 fluence on its general aspect remarkable. 



But the rocks themselves, to which I am thus desirous of limiting 

 the name of syenite, are so associated in their habits and formation 

 with the rocks of the trap family, and with those porphyries which 

 appear to have originated in similar circumstances, that they have 

 a strong claim on us for one generic term, as well from their geo- 

 logical, as their chemical, or mineralogical relations. If then we 

 consider the term syenite as founded on this double view of the 

 nature and position of the rocks to which I allude, it will be ob- 

 viously convenient, if not absolutely necessary, to separate from 

 them the rocks of a granitic character; of these the most simple 

 in its construction is that which consists of quartz, felspar, and 

 hornblende, or of those three ingredients with mica. Such rocks 

 are not of uncommon occurrence, nor is it uncommon to meet a 

 rock in which quartz, felspar and mica are so compounded, as to 

 form a true granite with crystals of hornblende superadded to the 

 compound. Great confusion must follow the separation of these 

 from the granites, and their union to the syenites above described. 

 Their structure and characters are in all cases granitic, nor does the 

 eye readily detect the difference till the darker parts are minutely 

 examined, when the crystals of hornblende may, and that often 

 only with much care, be distinguished from the mica. Their geo- 

 logical affinities associate them also with the granites, and never with 

 the greenstones, since they occur in \.\\q formations of the granite 

 sera, as hornblende rock and hornblende slate are known to do. The 

 obvious distinction would be to call these compounds syenitic 

 granites, thus preserving their analogy with the syenitic greenstones, 

 and preventing us from confounding into one mass two formations 

 of widely different characters and epochas, merely from the acci- 

 dental presence of a single ingredient. 



