found in the Marble of Carrara. 25 



are generally remarkably clear. Spallanzani was satisfied from 

 those in the museum of Pavia, where there is a great number 

 of specimens, that they surpass in limpidity the purest crys- 

 tals from Germany, Hungary, and Switzerland. 



The largest and most perfect of these crystals are contain- 

 ed in irregular cavities of the calcareous mass in the crystal 

 ovens, as the workmen call them, (forni a cristalli,) per- 

 fectly closed on all sides. Here the crystals are insulated, 

 sometimes in groups, but always adhering to the marble. 

 Most frequently they are found implanted perpendicularly to 

 the sides of the cavities. Sometimes, however, their pyra- 

 midal extremities are free, and they touch the rock only by 

 the faces or angles of the prism. 



The small crystals which are encased in the substance of 

 the marble have no transparency. Their colour is milk-white, 

 and their exterior form is not regular. One might suppose, 

 says M. Repetti, that want of room has also prevented them 

 from assuming the geometrical forms of crystals contained in 

 cavities. 



Rock crystal is never found in the statuary Carrara marble. 

 It occurs in the common white pearly marble of the grottos of 

 Coloinbara delta Paiastra and the Fossa deW JngelOy situated 

 near the foot of Monte Sacro. 



The workmen employed in the Carrara quarries informed 

 M. Repetti, during his first visits, that the cavities in the 

 marble which contained quartz crystals generally contained a 

 greater or less quantity of pure water, slightly acidulated ; 

 that they have often recourse to this fluid to quench their 

 thirst ; and that the crystals of calcareous spar encased in the 

 substance of the marble, and which they call luciche, are al- 

 most a certain proof that a liquid cavity containing quartz 

 crystals is not far distant. Hence the workmen have called 

 these crystals spies (la spia.) M. Repetti has satisfied him- 

 self of the accuracy of these observations. 



I proceed now to the extraordinary fact which forms the 

 principal object of this paper, 



" In the spring of 1819, M. Pontaleone del Nero, proprie- 

 tor of a quarry in the Fossa del Angelo, having caused to be 



