ON GEOLOGY, 



The real volcanic minerals are those which have been thrown out of 

 the crater of a volcano, and consist of three kinds : first, those which, 

 having been discharged frequently, have formed the crater itself of the 

 mountain ; secondly, those which have rolled down in a stream, and are 

 known by the name of lavas : and, thirdly, the residual matter contained 

 in the water which is often ejected, composed of ashes and other hght 

 substances, and which, when rendered sohd by evaporation, is denominated 

 volcanic tuff or tufa. 



I have observed, that these different classes of mineral formations are 

 often traversed in various directions by other mineral substances which are 

 called VEiNs^ as if the rocks they compose had split asunder in different 

 places from top to bottom, and the chasms had been afterwards filled up 

 from other sources. These transverse lines or veins are worthy of notice 

 in regard to their shape and the ftuhsttances with which they are filled. 



With respect to their shape^ they appear to be almost always widest 

 above, and gradually to diminish as they deepen, till at last they terminate 

 in a point ; exactly as if they had been originally fissures in the rock. ^ 

 Occasionally, indeed, they are observed to widen and contract alternately 

 in different parts of their course, but this is by no means a common ap» 

 pearance. 



Sometimes they are partially or altogether empty ; atid in this case they 

 are real fissures, and are so denominated ; but generally they are filled 

 with matter more or less simple, and more or less different from the rock 

 through which they pass. All the formations I have already noticed as 

 existing in the shape of rocks have also been found in the shape of veins : 

 whence we have veins of granite, porphyry, limestone, basalt, vvacke, 

 green-stone, quartz, clay, felspar, pit-coal, common salt, and metals of every 

 kind. When the veins are compound, or consist of a variety of substances, 

 these substances are almost always disposed in regular layers ; one species 

 of mineral constituting a central line or cylinder, and this being incrusted 

 with a second mineraV, and the second with a third, and in the same manner 

 to the utmost sides of the veins. These layers are occasionally very nu- 

 merous ; that of the vein Georgius at Freyburg consists of not less than 

 nine, and there is another in the same district, which, according to M, 

 Werner, extends to thirteen. It is not uncommon to find veins crossing 

 each other in the same rock ; and when this occurs, one of the veins may 

 be traced passing through the other without any interruption, and com- 

 pletely cutting it in two, the cut vein always separating and vanishing at 

 the point of intersection . 



Nothing appears more obvious than that these veins must have been 

 originally fissures produced by some unknown violence in the rocks in 

 which they occur ; and it is highly probable, as conjectured by M. Werner, 

 that the mineral materials which constitute them have been deposited 

 slowly fi-om above during the formation of the different classes or sets of 

 rock of which the different layers consist, while the rocks in which they 

 occur were covered with water. Upon this theory veins are of course 

 newer than the rocks in which they are met with, and which must have 

 split to have produced them : and where two veins cross each other, that 

 is obviously the newest that traverses the adjoining without interruption, 

 as the fissures constituting the second vein noust have been formed after 

 the first was filled up. 



The FIVE classes of rock formations we have thus far considered arc 

 * those which entered into Professor Werner's system, as it first made its 



