A little sedimentary agate info- Dryheads, Fairburns etc

Thanks Mel, very interesting to read.
I appreciate you taking the time to post this!
I've never dug for Dryheads, but I can tell you that the chert matrix on the Teepees is some pretty hard stuff!
As far as the Fairburns go, I've never found them to be in a "standard" matrix (as jasoninsd stated). Sometimes I've been fortunate to find them with no matrix at all but usually they are in some sort of host rock. Which I guess would actually be considered a matrix.
I have all three that I'll try and get some closeups of today and post to this thread if I can find similar patterns- if you don't mind?

Here's some pics of each of the three...

Roger Clark , in his first book stated Fairburns have been found in situ in host rock in the black hills. The matrix of Fairburns has often been referred to as jasper but to me, chert matrix, like TeePees and Dryheads, makes more sense as chert arises from sedimentary formations whereas jasper is more in igneous deposits. It was proposed that TeePees, also being from Minnelusa Limestone, are actually just a variety of Fairburn that is in situ. If you look at the pics in Clark's book, boy some of the Fairburns do bear a strong resemblance the Tee Pees.

Great info and pics.
It seems like these agates share some similarities with crazy lace, when it comes to their formation.

I think that is backwards Stephan. I believe agates form at low pressure and temperatures and they form quite fast. ( fast being hundreds of years of course) Regarding the waterline type formations, that is an interesting question and I'd have to do some serious reading to try and answer that one. I don't recall seeing a lot of waterline formations in sedimentary fortification agates like Dryheads etc, but they are quite common in t-eggs and gas pocket type nodules like Brazilians, Bots, Queenies etc found in basalt. And yes, crazy lace is found in limestone or dolomite so is sedimentary in nature.

OK read several theories with lots of experts arguing over agate formation. Seems as though one theory states waterline agates occur in volcanics because "vibration" makes those layers settle first and as it diminishes the other layers form around the edge. * scratching head*. OK if they say so. Theories say alkaline conditions are required to carry silica into voids from overlaying volcanic ash deposits. Perhaps we don't see waterline layers in sedimentary agate because the cooling action found in volcanics is not present in sedimentary layers so no vibrations to make initial silica settle to the bottom of the nodule thus making the layers form continuously around the edge of the cavity being filled.

Or , maybe I'm confused after reading all these theories and have boiled the information down all wrong *L*. Lenz's agate books go into agate formation in excessive detail if any of you want to find one and give it a read. I just prefer chemistry made purdy rocks for us to find.

Oh, each new filling can re-dissolve some of the previous crystals.

Many years ago I dug into a wet agate seam that fell apart in my hands.
I dug a trench and let it dry and it hardened. That experience left me puzzled for many years.

A wet agate seam? Wow! That has to be a once-in-a-lifetime experience.  Educational and sad to have it fall apart, all at the same time.

Thanks for clarifying.  The full vs. partial filling makes 100% sense.  So seam agates with lacy, but not fortification agate could (maybe?) be formed similar to leveling lines, but with something like ground motion to perturb them when they are mostly, but not entirely solidified....  Interesting stuff.


You'll have to excuse me.  I think I have an extra copy of the "Why" chromosome.