Our granite plateau is known as the Strathbogie batholith, and extends from East of Whitehead’s Creek nearly to the Broken River In the past, many millions of years ago, the surface of this earth was several kilometers above the present surface and was all under sea. The granite was extruded from great depth, and came up in a porridgy mass through the marine sediments. As it solidified it cooked the surrounding sedimentary layers (sandstone and mudstone) and made them very hard, locally known as “ironstone”, but geologically is “hornfels” – a metamorphic rock. Because this is so hard it has been retained as high ridges on the edge ofthe granite, e.g. the Switzerland Range between Highlands and Yea. As the sedimentary rocks were eroded away, some bits fell in to the molten mass. I once found a granite boulder on the top of Wattle Hill, with a very recognizable bit of sedimentary sandstone still in it. Granite is composed of 3 minerals – quartz, feldspar and mica and other accessory minerals such as tourmaline. Here it is black, but can be transparent red or green, in which case it is of gem quality. As the granite cooled, it crystallized and very good quartz crystals can sometimes be found. Mostly these are opaque, due to trapped air particles, but if clear, they again are gem quality. If radioactivity is present, quartz crystals can be quite black and then are known as “Morion”. ”Citrine” is another variety which is a browny-yellow, and makes very attractive jewellery. Topaz is another gemstone found in association with granite. Sometimes, during the cooling process, vertical “pipes” formed underground, and crystals could grow on the walls. The most notable that I know of, are at the Crystal King Mine at Strathbogie. Now, these shafts have been fenced off, but about 50 years ago, you could gaze down them and actually see the crystals.
These notes were prepared by Peg Lade of Highlands and Published in the Granite News 2008