250 Outlines of Geology. 



which geologists are somewhat at variance. Hornblende, or 

 Schiller- spar, and felspar, appear to be essential to its constitu- 

 tion, and perhaps also talc ; and these three substances are some- 

 times distinctly visible, like the three components of granite ; 

 while, at others, the rock is so fine-grained as to be nearly homo- 

 geneous ; its variety of colours recommends it as an ornamental 

 material, but it is too soft to admit of much polish. The bright- 

 green varieties, interspersed with white marble, form the verde 

 antique, to which some of the serpentine of Anglesey bears a 

 resemblance ; and those varieties which are comparatively hard 

 and brilliant in their colours, are called noble serpentine. Talc, 

 which I have mentioned as a component of serpentine, is lamellar ; 

 but its lamina, unlike those of mica, are not elastic, and it has the 

 soft soapy feel of a magnesian fossil. In composition it differs 

 little from the steatite, which is found pervading serpentine in 

 veins, [and which, in many parts of the Lizard serpentine district 

 in Cornwall, is accompanied by veins of a rock sometimes looking 

 like the massive felspar, and at others becoming porphyritic, or 

 even assuming the characters of granite. These are very curious 

 facts, and it requires some forbearance to pass them over merely 

 as such, -without considering the theoretical views to which they 

 lead. 



I can hardly be induced to regard marble, serpentine, porphyry, 

 or even syenite, as distinct formations. Sometimes they appear 

 as beds or detached masses, varying in magnitude and extent ; 

 at others they look like modifications of some other rock. Quartz 

 rock, too, which is said to form entire mountains, as in Jura, in 

 Wicklow, and elsewhere, and which are characterized by their 

 conical form and insulated appearance, is seldom free from mica, 

 and often may be traced by regular gradation into mica-slate, and 

 this into gneiss and granite. But I throw out these observations, 

 not with the intent of speculating upon them, but merely to sim- 

 plify the history of primitive rocks, and to shew that the occa- 

 sional recession or addition of one ingredient may give the whole 

 mass a new character, mineralogically speaking, without entitling 

 it to rank as a new formation, geologically considered. 



