154 MR GOODCHILD. 
The Shapes of Agates are determined by somewhat com- 
plex causes—mainly by the degree of liquidity, or of vis- 
cosity, of the lavas, the degree of pressure exerted at the 
platform in which they are found, the rate of flow of 
the lava, and by some few other causes. In a few in- 
stances a forward movement of the rock has been made 
after the enclosing walls of a cavity have consolidated. In 
such cases the cavity may be sheared in two, with its sepa- 
rated portions not quite conformable to their counterparts. 
When such a cavity becomes filled with an agate this appears 
to be faulted. Some very remarkable examples of these are 
to be seen in the Scottish Mineral Collection in Edinburgh ; 
while Mr Thoms of St Andrews, and Mr R. Miln of the 
same city, possess examples equally good. 
Taking it for granted that the chalcedonic materials of 
agates represent silica which has been carried into the closed 
vapour cavities of eruptive rocks by aqueous action, we 
have next to inquire where that silica came from, and how 
it found its way into solution. 
The first point to remember is, that eruptive rocks 
consist mostly of natural silicates, that is to say, of various 
bases—alumina, magnesia, lime, soda, iron, etc., in combina- 
tion with silica. Chemical action, which breught about the 
union of these, is competent also to effect their liberation. 
One of the most important agents concerned in bringing 
about such changes of composition is water weakly charged 
with various compounds. The solutions, being usually very 
weak, take a correspondingly long time to act. Of these 
solutions one of the most important originates in connec- 
tion with the products of decomposition of organic matter 
at the surface. Plants, under the stimulating action of 
sunlight, extract carbon-dioxide from the atmosphere, and 
eventually fix it as one or other of the carbon compounds in 
the solid form of their own tissues. When the plants die, 
they are at once attacked by bacteria, which end by con- 
verting the plant tissues into other compounds of carbon, much 
of which eventually goes back again into the atmosphere in 
the original form of carbon-dioxide. At the intermediate 
stage in the process of conversion, certain organic acids are 
formed. These are the Humus Acids. They are readily 
