GeologyRocks

An argillite will contain layers (laminations) around a few mm thick. It may also contain fossils. The basalt will be crystalline (may just be viewable with a handlens), may have phenocrysts, will not have fossils or layers, may have vesicles.

In thin section it will be very obvious due to the different mineralogy and textures.

That's all I could think of from the top of my head.

I have aboslutely no idea about the other two!

Jon

Thanks Jon.

I found an answer about the dam construction etc.
(The property of schist that may affect its suitability for a dam construction is its `schistosityÂ’ (the tendency to split along cleavage surfaces). Areas underlain by schist are prone to slope instability due to the planes of weakness caused by `schistosityÂ’.
The property of marble that may affect its suitability for a dam construction is itÂ’s `effervescenceÂ’ (Solubility in weak acid can be a hindrance). )

But still cant get an answer to why quartz and calcite might affect the coherance of greywacke.

Thanks Again.

Hi Warren

You came up with the same answer that I would have regarding schist and marble. As far as the calcite and quartz question is concerned it's not as obvious:

There's a hardness difference, quartz being much harder than calcite. The latter also is attacked by acid, such as in rainwater or peaty soils. Therefore, when weathered, the calcite veins will tend to dissolve and could lead to a lack of coherence in the greywacke, wheras the quartz will be stronger than the greywacke and will tend to stand proud of the host rock. In addition, although this may not be part of the answer, I have noticed that when quartz veins occur in greywacke, the quartz actually impregnates the rock to a degree and leads to a general hardening of it, whereas clacite doesn't lead to much impregnationa and subsequent alteration.

Gus

I'm really impressed with that Gus!!!! :shock: