Drilling into granite and river rock

Quoted text begins.I am attempting to anchor a large river rock to the granite for the use
of a chair
End of quote.

Jim,

You could try drilling into your rocks with a 0.5" masonry bit just to see how
hard the stones are. Extended hammer drilling with a jaws chuck will destroy the
chuck. Ideally you should use an SDS+ or SDS Max drill.

1-1.5" hole and dowel size is huge. Are you sure you couldn't use three 10mm
dowels instead of one big dowel? Two dowels at least will prevent the rocks from
turning about the fixings.

Regarding epoxies: almost any general purpose gap-filling adhesive like Araldite
will do. The strength of this type of arrangement is in the location of the
dowels, and relies on the shear strength of the steel - preferably stainless
steel, but you could also use zinc-coated rod. If you are concened about
adhesion, make sure that the holes are clean and dry, 'prime' them with the
epoxy by twisting the rods around in the epoxy-filled holes to pick up any
remaining dust, and use "Booker' (threaded) rod for the dowels.

Simon