108 OVERLYING AND TRAP ROCKS. 



containing no iron. All the grey varieties owe their 

 colours to the protoxyde ; as intensity of colour is a 

 sufficient test of its proportions. From the peroxyde 

 ure derived the varieties of red and brown ; and it is 

 worthy of remark that the porphyritic veins found in 

 granite, are generally characterized by these colours, 

 while grey or black predominates in those that occur 

 in the secondary strata. 



The term BASALT has been hitherto applied to every 

 'trap rock characterized by uniformity and minuteness 

 of texture, united to a black colour ; but these do not 

 form a mineralogies! species. Thus it has included 

 the dark and hard claystones and clinkstones, and the 

 greenstones of a minute intermixture ; while, as I 

 have ascertained, the fine augitic greenstones, together 

 with pure augit in a minute condensed crystallization, 

 have equally been confounded under this term, by the 

 multitude, more ready with names than acquainted 

 with minerals or rocks. Let it at least be a definite 

 rock ; and as all the others are classed, it should be 

 limited to a minute aggregate of hornblende alone. 



GREENSTONE has been supposed to include the dark 

 coloured mixtures of hornblende and compact felspar; 

 but I have just said that colour is not a true ground 

 of distinction between them and Syenite. But the 

 very term itself is the produce of that mineralogical 

 ignorance which I have tried to correct by the esta- 

 blishment of Augit rock. This is often a greenish 

 substance, but the mixtures of felspar and hornblende 

 never are so : the reputed mineralogist who has mis- 

 led all that ever believed in him, appears to have con- 

 founded augit and hornblende. Greenstone must 

 now mean the mixtures of hornblende, yet not with 

 compact felspar alone, but with clinkstone and com- 

 mon felspar also, equally overlooked ; while it is 



