HORNBLENDE ROCK AND HORNBLENDE SLATE, 



79 



Gothard, and in the serpentine at the Lizard in Cornwall. For a 

 considerable time, it was unknown where saussurite occurred in situ; 

 it has since, been discovered in immense beds, associated with ser- 

 pentine, in the valley of Sass, in the Haut Valois. Near Nyon, on 

 the Lake of Geneva, one hundred and twenty miles distant, there is 

 a field, over which are scattered large blocks of the same stone, and 

 on account of their unconquerable hardness, the proprietor has been 

 unable to remove them by blasting. Beds of saussurite occur on 

 the southern side of the Alps, and in the Appennines. A very inter- 

 esting description of the saussurite and serpentine of the Appennines 

 has been published by M. Brongniart, entitled Sur le Gisement ou 

 Position relative des OphioIiteSj Euphotides, et Jaspes, dans quel- 

 qiies Parties des Appennines.^ In these mountains, the serpentine 

 rests upon saussurite, the saussurite on strata of jasper, and the latter 

 on secondary limestone. This position is remarkable, for geologists 

 had long supposed that all serpentines were more ancient than the 

 secondary rocks. It has, however, been recently discovered, that 

 some trap-rocks which are in contact with beds of limestone, or cut 

 through beds of limestone, are changed into serpentine, apparently 

 by intermixture with calcareous earth. This discovery throws much 

 light on the true nature of serpentine : we can no longer be surprised 

 at finding these rocks in formations of different epochs. Though 

 serpentine may, in many instances, be considered as a rock whose 

 quality has been changed as before stated, yet it would be contrary 

 to sound induction to maintain that serpentine may not, in other in- 

 stances, be an original rock formation. Wherever the earths that 

 compose serpentine have occurred together in due proportions, the 

 same causes which have produced other mineral combinations, may 

 have formed serpentine : it is rendered almost certain that this has 

 been the case, as many rocks containing chlorite and hornblende, 

 appear to pass by gradation into serpentine. 



H.ornhIende-Rock and Hornblende- Slate. — This mineral has been 

 described Chap. III. When it forms the principal parts of rocks, 

 the colour is commonly a greenish black. Massive hornblende, in 

 rocks, is generally coarsely granular and lamellar ; in hornblende- 

 slate, it is frequently radiated or fibrous, and when the fibres are 

 very minute it has a velvet-like lustre. Hornblende-slate occurs in 

 beds in granite, gneiss, and mica-slate, and occasionally in common 

 slate : it appears to pass by gradation into serpentine : the change is 

 effected by an increase of magnesia, which forms one of the con- 

 stituent parts of hornblende. 



Hornblende in large lamellar grains intermixed with felspar, forms 

 sienite, which it was remarked in the last chapter is not unfrequent- 



* It is to be regretted that so excellent an observer and mineralogist as M. 

 Brongniart, who is so justly eminent for his scientific labours, should have thought 

 It necessary to burden Geology with two additional new names. Serpentine he 

 has denominated ophiolite and saussurite euyhotide. 



