Posted: Sep 22, 2015 12:44 pm
by Adco
I was having a discussion with a guy who said he "dabbled" in geology when he worked for a mine. I don't fully agree with his explanation. It related to the rocks in my garden that have now been exposed to reveal something the caught my interest.

Here is a pic of the area. I removed about 30 tons of rock from the area to make a fire pit feature. How I managed is another story but it is now complete and we have had many wonderful nights sitting around socialising.

Fire pit.jpg

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You will notice that halfway up the large rock at the back is a "water line". This is the old level of the ground before the rocks were removed.

The exposed area had an interesting formation that become visible.

Dissolved quartz.jpg

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Imbedded in the bulk of the rock are many small white stones. We identified them as quartz. Notice that below the old level, the quartz has been dissolved. There are neat little holes where the quartz once was. I am guessing that rain water seeped down along the rock face and slowly ate the quartz away. The main part of the rock that was also below the ground level is darker and softer than the rest of the rock. More like a sand stone feel to it.

My guess that water dissolved the quartz is based on the pic below which shows a small cavity, almost like a "fairy house" with shiny crystal formations. You can clearly see that the edges of the cavities are composed of what was a large quartz stone.

Fairy house 2.jpg

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Here comes the disagreement between the geologist and myself. I say that the quartz pieces that we can see embedded in the main bulk of the rock were collected as the molten rock flowed over an area that had small stone lying on the ground. Much like raisins in a bun. He says that they grew in place from water flowing through cracks in the rocks. Another option that I can accept is that this is sedimentary rock that had quartz deposited in some unknown way.

I disagree because I don't see any cracks where the water could have flowed. It is too homogenous. The quartz stones are tightly packed. There would have been gaps of some sorts where all this water was supposed to have run. Also, the cavities are formed by water dissolving the quartz. If water can dissolve the quartz, how is it going to deposit it so tightly into the rock in a growth method. The holes where the quartz used to be, below the ground level have been totally dissolved. Like someone picked the raisins out of the bun.

I am not too sure what type of rock this is. My guess is basalt. The above is assumption only. Who knows about this stuff?