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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NOV. 6, 



muddy matter was separated from the coarser sandy portion by sus- 

 pension in water and decantation ; from which I found that we had 

 in 100 parts — 



Matter soluble in hydrochloric acid, being chiefly calcareous 60 



Coarse sandy matter 34 



Fine sandy matter 6 



100 



This coarse matter consisted, in nearly equal proportions, of the re- 

 niform agatized bodies, and sand, whose particles varied in size from 

 yi^th to 2Wo tn °f an mcn > tne average being y^-th, of which the 

 great bulk was composed. The fine matter contained some few grains 

 of y-i-oth to y^Q-^th of an inch, and all sizes down to ioiWo tn or ^ ess > 

 but the bulk was made up of those of from i^th to ^ooth of an inch. 

 Hence it is clear that this deposit was not subjected to much wash- 

 ing at the time of its deposition, or else it would not have been com- 

 posed of particles of such very varying magnitude. 



Although in some parts the coarse sand and reniform bodies occur 

 in such numbers as to touch one another ; and although some por- 

 tions of the grit retain their figure when the calcareous matter is dis- 

 solved away, yet in others it may be seen that a considerable part of 

 it was deposited at the same time, and was not merely infiltrated 

 among the coarser particles subsequently, although much of it has 

 undoubtedly been introduced in this manner, If a thin slice of the 

 stone be prepared, not much thicker than T oVotri of an inch, and 

 examined with a microscope, the manner in which the grains of sand 

 and the reniform bodies are imbedded in the impure calcareous de- 

 posit is well seen. The form of these bodies, however, is best made 

 out by examining them as left after dissolving a portion of the stone 

 in acid, when it will be seen that they vary somewhat in their shape. 

 In many cases they are of a form that may most truly be termed 

 reniform, one side being perfectly rounded, and the other having an 

 incurved depression of varying depth, in some so slight that the body 

 is almost globular. When turned round, they are seen to be more or 

 less depressed, and to have an oval form, both sides being symmetrical. 

 Their size varies from T y^th to ^^th °f an inch, the average being 

 about y£-oth. Their internal structure is best seen by mounting 

 them in Canada balsam, which, having nearly the same refractive 

 power as the agate, causes the light to pass through them as regu- 

 larly as if we had a thin section. I have also carefully investigated 

 it by examining them in the latter condition, as occurring in a thin 

 slice of the stone. By these means I find that they often have a 

 structure similar to that frequently seen in large agates, the deposit 

 having begun from the sides, and left a vacancy in the centre, which 

 was afterwards filled up with less pure agate in alternating, more or 

 less coloured strata. It is only in some cases that this sort of struc- 

 ture is well seen by transmitted light ; but when a thin slice is ex- 

 amined by reflected light, it is seen much more frequently, owing to 

 the circumstance that many, in which it is really present, are occu- 



