SIR G. S. MACKENZIE on the Formation of Chalcedony. 93 



quartz, we may appeal to it for illustration. Let us suppose, 

 then, the cavity placed with its two ends in the direction of the 

 plummet, and fluid Chalcedony entering by the upper end, and 

 obeying the law of gravity. In these circumstances, I cannot 

 conceive how the fluid was conducted, so as to coat each crystal 

 uniformly on all sides, and at the same time so that no more 

 Chalcedony should rest on the lower than on the upper faces and 

 angles of the quartz-crystals ; nor can I account for there not ha- 

 ving been a mass at the bottom with a horizontal surface. Thus, 

 when we examine a specimen with minute attention, we find our 

 first notions respecting it incorrect. With these remarks, I leave 

 this remarkable specimen, as a proof of the danger of maintain- 

 ing sweeping theories, that pretend to explain every thing. It 

 has been asserted, that a philosopher sitting in his closet, with a 

 specimen before him, cannot reason correctly on the mode in 

 which a mineral has been formed ; but the specimen we have 

 been considering proves the contrary ; and also defies him who 

 may boast of having traversed every mineral mass that is exposed 

 to view anywhere on the surface of the globe. We may be 

 enabled accurately to ascertain general circumstances ; but par- 

 ticular modes of action, and formation among mineral substances, 

 under such circumstances, we can explain but seldom. 



I now come to consider the Pendulous form of Chalcedony, 

 which affords an ample field for the exercise of theoretical inge- 

 nuity ; and at the same tune proves the utter hopelessness of the 

 task, when any one undertakes to find in it a demonstration of 

 his theory, and attempts to discover the minute circumstances, 

 and laws of action, which produced the forms which he studies. 

 The forms we have already considered, do not afford sufficient 

 data for assuming either the kind or degree of fluidity to which 

 Chalcedony may have been subjected ; but we shall find them in 

 the pendulous form. 



