

« 



f 



t . 



P 



I 



I 



ft 



t 



Sect. VIII.] 



MINERALOGY, 



253 



I 





1 collecting mmerals in a vein, should a boat be 

 hand, so that they may he readily taken to the ship, it 

 is better not to Hmit the specimen to some mere crystal 

 itself, but to break off some of the body (either part of 

 the vein or of the rock, as the case may be) upon which 

 it has been formed, so that when more leisure may be 

 obtained, any illustration the whole specimen may afford 

 of the manner in which the mineral may have been formed, 



should be preserved. By such specimens we often learn 

 the history, as it were, of the mineral accumulations 

 which, taken together, may, wholly or in part, have filled 

 up a fissure. Tn this way it may often be seen that crys- 

 talline coatings of many substances have successively co- 

 vered each other up towards the centre of the fissures. 



The contents of veins are far often from heir 



defi- 



nitely crystalline ; thus qiiartz and other mineral sub- 

 stances, such as the ores of many metals, have an amor- 



phous appearance, their deposit having been effected 



under conditions which did not permit their particlefe to 

 adjust themselves in definite crystalline forms. 



A-gain we 



find that, during the filling up of veins, fragments of 

 rocks from the sides or upper parts of the fissure have 

 dropped in ; by their want of contact and by their iso- 

 lation in many parts 



of the vein sliowlng that 



thl 



happened when the mineral or minerals thrown do^vn 



from solutions were accumulating. 



A 



mm 



era! 



3f- 



vein 



sometimes forms a complete breccia, and this as well 

 from the cause just assigned as from the mere filling up 



of the chinks left by fragments from the adjoinin 

 accumulated in the fissure before any deposit of minc.dl 

 matter from solutions was effected. As might be ex- 



M 



k 



