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    It's obvious that the solutions need some kind channel to enter and fill up the agate lined cavities in order to keep building up the agate a layer by layer. Different people have different ideas on how a channel formed, hence, different names for the "channel" like "Entrance Tube", "Fill Tube", "Escape Tube", and "Infiltration Channel". I thought that it would be easier to just use the term "Channel" to describe such tubes. The idea of "closed system" is that the pressure would build up as a result of water being compressed by growing but not yet sodified agate layers and eventually, the pressure would cause solution to part the soft agate layers apart. The problem is that I can't see how can the agates and especially quartz that have already crystallinzed SOLID be deformed as well as the fact that the agates have conducts that could relieve the pressure within the cavities. The coloring impurites and some minerals inclusions such as calcite and geothite situated between the agate layers also seriously challenge the idea of "closed system".
     The "Open System" idea is that the flow of solutions builds up at leat one channel as the agate layers were deposited over time just like a fan blowing a lot of dust thru a small vent into a room where the layer of dust would be thinner toward the fan while thicker toward the other side of the room. Also think of a drain pipe where water is flowing out into rivers/lakes. If one looks at such drain pipe during winter where water is still flowing, one would notice that there're very little ice at the mouth of the pipe while there are plenty of ice on the other side opposing the pipe. If the flow of water is interputted periodically, the successive layers of ice would form until it reaches and clog the drain pipe. I have a pond with a founatin at one end that is behaving excalty what I described and sometimes teh channels had been formed by flowing water. The idea of "open system" seem more plausible since it would allow the enviromental factors such as climate, water and mineral leeching to influence the formation of agates and produce different colored layers and mineral inclusions. The nature of the complex channels may challenge the idea of "open-system" idea since the channels tend to come very complicated forms (such as winding "worm tubes") with seemingly deformed channel walls that seem to support the idea of "Closed System".