imbedded in a Limestone near Cork. 1 23 



most part, and sometimes are considerably bright ; they are 

 externally of that colour which is termed smoky or brown 

 quartz, and may generally be separated from the limestone 

 readily, leaving a cavity exhibiting their precise form. 



In order to observe as accurately as possible the interior 

 of these crystals, I attempted to break them across the prism, 

 at right angles to the axis. I found, 

 however, tliat they would yield only in 

 directions parallel, or nearly so, to one 

 or other of the planes of the pyramid, as 

 is the case with common quartz. The 

 annexed sketch exhibits a cleavage in that 

 direction, and, generally speaking, also 

 the aspect of the plane produced, which 

 appeared to consist of alternate and con- 

 centric prisms of smoky quartz, which 

 in thin fragments is highly translucent, 

 and of gray, opaque, and somewhat gra- 

 nular limestone. 



On applying a drop of diluted muriatic acid to the fractured 

 surface, a brisk effervescence immediately ensued along the 

 parts which are of a gray colour, satisfactorily proving the 

 presence of limestone, and inducing a hope that it might thus 

 be possible to dissect the crystal, by separating the smoke-co- 

 loured layers of quartz from each other : but in this I was dis- 

 appointed ; for the effervescence soon ceased, and I could 

 not jjrocure a renewal of it for any considerable time, by re- 

 placing the first by a second quantity of diluted acid ; and the 

 result of leaving the fragments in it for a week or two was, 

 that the gray parts of the crystal became visibly cellular, and 

 so soft as to admit of being scraped away by the point of 

 a knife, leaving a little channel between the lines of brown 

 quartz. 



The annexed sketch, though not an absolutely faithful por- 

 trait of a crystal in my possession, shows the fact, that the whole 

 consists of a minute central crystal of quartz surrounded by 

 four thin layers of the same mineral, separated by a gray sub- 

 stance constituted in part of limestone. The outer coat of 

 brown (|uartz is generally thicker than the layers within it, of 

 which however the sides towards the exterior of the crystal 

 are almost always better defined than the interior, which min- 

 gle with the gray substance, so as sometimes to prevent the 

 occurrence of a complete line of separation between them; 

 and the lines of brown quartz arc not always so perfectly de- 

 fined as is representetl in the sketch. I have observed several 

 crystals in which the lines of (juartz are far more numerous, 



]i 2 and 



