1 869.] Mineralogy. 305 



is a question still open to discussion. Some suppose that the 

 solution transuded through the walls of the entire cavity, whilst 

 others argue in favour of a local infiltration confined to special 

 inlets, which are often exposed on cutting a section of an agate. 

 Probahly both theories may be advantageously combined ; the de- 

 posit of "green-earth " which commonly lines the smaller cavities 

 being due to a general percolation through the walls, whilst the 

 solution of silica was introduced through definite channels or inlets 

 of infiltration. It is difficult, however, with either theory to 

 explain satisfactorily the formation of a regular succession of con- 

 centric layers lining the internal walls of a cavity, aud not deposited 

 simply upon its floor ; whilst it is still more difficult to imderstand 

 the conditions under which concentric layers were deposited at one 

 time and horizontal layers at another — conditions which must have 

 obtained during the formation of many of the so-called " Brazilian 

 agates," which exhibit both deposits in the same stone. According 

 to Keusch, the successive layers have been deposited from a solution 

 which was alternately forced into and expelled from the cavities by 

 the action of intermittent thermal springs. A modification of this 

 theory is that adopted by our author. He supposes that the solution, 

 after having deposited gelatinous silica in the cavity, was heated to 

 its boiling point, and that the elastic force of the steam thus 

 generated would press the siliceous jelly equally against all sides of 

 the cavity, so as to produce a layer of uniform thickness throughout. 

 The compressed steam would then slowly make its exit by per- 

 forating a passage through the successive layers ; but if this outlet 

 became sufficiently large, the steam might escape before acquiring 

 sufficient tension to keep the gelatinous mass distended against the 

 walls, and hence the silica, left to the force of gravity, would be 

 deposited in horizontal layers upon the floor. In this way the 

 author attempts to explain the alternation of concentric and hori- 

 zontal deposits in the same cavity — a point which has hitherto been 

 a standing enigma in all theories on the formation of agates. 



The occurrence of beautiful agates in the melaphyre of the 

 Saarbruck coal-field led at an early period to the localization of the 

 agate trade in the neighbourhood of Oberstein and Idar. Although 

 for many years past no agates have been quarried in that locality, 

 all the fine stones being imported from Uruguay, the trade of 

 cutting and polishing still remains active, and at the present time 

 there are 183 mills in the district, carrying 724 grinding stones; 

 each double mill being reckoned as two single ones. 



In the Chronicles of Mineralogy for July last, attention was 

 directed to a remarkable shower of meteorites near Pultusk, in 

 Poland.* Many thousands, not to say hundreds of thousands, of 



* ' Quart. Journ. of Science,' vol. v., p. 419. 



