An excerpt from Nick Norman's book entitled "Geology Off The Beaten Track" and published in 2013:
Building Aggregate (page 28).
"You pass two quarries as you edge past Durbanville. In both, the stone is mined that is particularly suitable as aggregate, whether it's for road-making or for mixing into concrete.
Such stone should be hard, durable, should crush easily and should be located close to where it is needed.
Granite is hard, in most cases too hard. Shale on the other hand, although it is well bedded and often closely jointed, is too soft even when fresh.
Worse still, on contact with air and water, it quickly decomposes to ultrasoft clay minerals, making it completely unsuitable.
The irony is that interaction between too-hard granite and too-soft shale - as it happened when the Cape Granite intruded the Malmesbury Group shales – gives us an ideal material.
We call it hornfels or baked shale or slate. The cooking was barely enough to change the appearance of the shales, but sufficient to turn them into good, hard slate, which, unlike its progenitor, resists breakdown when exposed to the elements.
So even if we don't see granite in the Durbanville Hills, it is close enough to the overlying sedimentary rock to have been converted to near-perfect aggregate material."