Singing rocks of Stob Ban

Thermal metamorphism (by contact with igneous intrusions) can cause very intense recrystalisation of minerals, but since the duration of heating is short grainsize remains small you end up with very well-bonded, hard and elastically stiff rocks. This can be especially true of carbonate-bearing rocks in proximity to granites where wholseale replacement of carbonate for silica can make quite exotic rocks, called skarns.
Here on the North-East flank of Stob Ban there is a thin band of 'limestone' which sits in slate - this boulder shows the wffects of contact metamorphism on the two rock types. A hammer blow is a good way to tell how well bonded the grains are - the better bonded - more perfectly elastic theock behaviour so the brighter the sound.

Check out this YouTube video of a lithophone made from contact-metamorphic rocks from near Keswick:
   • Xylophone Musical Stones (1936)