MINERALOGY OF THE REDHEAD, 



363 



respect differing from a true vein. It was about 

 six inches in breadth, and could be traced for 

 several yards. 



The vesicles of the amygdaloid, are frequently 

 very small, but in general they are from half an 

 inch to three inches in diameter, of a globular 

 form, and in some instances compressed and club- 

 shaped. These lengthened vesicles observe no 

 particular direction in the bed, being some- 

 times parallel, and at other times perpendicular 

 to its surface. When the rock is soft, these 

 vesicles occur in greatest abundance ; where it is 

 very hard and dense, they are seldom observed. 

 These vesicles are filled vv'ith various substances, 

 as limespar, quartz, flint, calcedony, jasper, and 

 green -earth. This last-mentioned substance of- 

 ten fills up entirely the cavities^ at other times on- 

 ly partially. ()uartz and calcareous-spar frequent- 

 ly occur together ; and the form of the crystal- 

 lization of the calcareous- spar is generally im- 

 pres^d on the quartz. Calcareous-spar and calce- 

 dony are often associated together in the same 

 cavity. These balls of agate are completely in- 

 sulated in the bed, and in general are sepa- 

 rated from the rock by a thin film of green 

 earth or clay, so that with a blow they are 

 readily detached. When in a state of decompo- 

 sition, the different concentric zones separate like 

 the coats of an onion, each exhibiting all the fea- 

 tures of a perfectly hollow sphere. 



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