394 Mr. C. Tomlinson on some Efects of small 



pebbles known as banded quartz. At one time I had several 

 hundred specimens, most of which I have distributed among 

 geological acquaintances. They consist of alternate bands of 

 crystalline and amorphous quartz, and the latter variety being 

 more porous and softer than the former, became abraded 

 during the rolling on the ancient sea-beach, so that the harder 

 crystalline portions project like the teeth of a saw. Now 

 there must have been a time in the history of this planet when 

 quartz was in a state of fusion, and it would probably contain 

 minute particles of matter entangled within it. Supposing the 

 temperature to have become sufficiently reduced for crystalliza- 

 tion to set in, the first layer in crystallizing would reject its 

 foreign matter into the second layer, and thus form two layers 

 of crystalline and amorphous quartz ; the process being re- 

 peated, we might have ten or a dozen alternate bands thus 

 formed, the breaking-up of the mass into smaller fragments and 

 their rolling into pebbles under the tidal action being subse- 

 quent events. 



Such a suggestion is of course inadequate to account for 

 the formation of siliceous minerals. Indeed, in the present 

 state of our knowledge, it is quite impossible to do so, and the 

 trivial names of the different varieties sufficiently point to our 

 ignorance. In the formation of the agates it is evident that 

 the successive layers were gradually formed at different times, 

 and the name of this mineral is said to be derived from the 

 river where specimens of it were found, namely, the Achates 

 in Sicily, now known as the Drillo, in the Val di Noto. It is 

 true that the carnelian is named from its resemblance to caro, 

 carnis, u flesh," as applied to the red variety, but this name 

 is not appropriate to the yellow. Onyx is so called from its 

 resemblance to the human nail, ovv^, unguis ; but sardonyx is 

 derived, according to some, from Sardesin Lydia, or from Sardo, 

 the Greek name for Sardinia, as chalcedony is from Chahedon 

 in Bithynia. One of the most curious derivations is a variety 

 of chalcedony of an apple-green colour, due to oxide of nickel, 

 known as chrysoprase, from xpvaeos, " beautiful," and irpaaov, 

 " a leek." Other siliceous minerals with trivial names might 

 also be referred to. 



Professor Graham read a paper before the Royal Society in 

 1864 on the properties of Silicic Acid and other analogous 

 colloidal substances, in which he says : — " The formation of 

 quartz crystals at a low temperature, of so frequent occurrence 

 in nature, remains still a mystery. 1 can only imagine that 

 such crystals are formed at an inconceivably slow rate, and 

 from solutions of silicic acid which are extremely dilute. 

 Dilution, no doubt, weakens the colloidal character of such 



