Abundant in Cornwall; however visually clear example.
Found at Condurrow mine - where there are a lot of elvan dykes in that
granite.
Breaking-up a rock cluttering a level between two vertical ladder
shafts - no way to get the rock out whole - the first split presented
this break face with this junction.
[Wikipedia links follow:]
The boulder being half
elvan
(fine, very hard) seen upper in pic. and
granite
(coarse, "just" hard) seen lower in pic.
The small white semicircular groove bottom of pic just right-hand-side
of centre is the drilled socket into which went the feathers-and-wedge
used to split the boulder - as described in
rock-splitting down the mine
.
Other granite-to-elvan contacts have appeared when splitting more
rocks while clearing during maintenance of Condurrow mine. That elvan
is indeed hard.
Every elvan dyke has two contacts; one on each side of the intrusion -
doubling the probability of a rock having such a contact.
(R. Smith, 12Sep2024)