tfh December ; 1883. 



The President, Dr. Cunningham, in the Chair. 



Mr. Joseph J. Murphy read a letter from M. Foster Heddle, on 

 AGATES. 



The following letter was addressed by the writer in 1871 to 

 Mr. Joseph John Murphy, and though not originally intended 

 for publication, is now submitted to this Society with the 

 writer's consent : — 



St. Andrew's, November 4, 1871. 



Dear Sir, — I have on my return found your note as to 

 Agates. Though I have been at work on the subject in different 

 ways for many years, I have not found myself in a position yet 

 to publish. In fact I cannot yet say that I know much as to 

 how they have been formed, though I do know, or rather am 

 able to show, that they have not been made in the manner 

 usually supposed. 



The late Principal Forbes conceived that they had been 

 formed by concentric deposition round a central nucleus: — this 

 I showed him to be untenable. Others conceive that siliceous 

 matter in a state of fusion has been poured into cavities through 

 an opening, such opening being called the " point of infiltra- 

 tion." I am able to show that this so-called point of infiltration 

 is an orifice of escape or exit of something. 



Fully to state how (from examination of their mode of occur- 

 rence, experiments upon the decomposibility of trap rocks under 

 the action of carbonated water, section of agates in every con- 

 ceivable direction, experiments upon their powers of absorbing 

 liquids, and from microscopic examination) I conceive agates to 

 be formed, would call for indeed a long statement. 



