At Penyrhiwan, Clynnog, in the middle of fields, is a protruding boss of dark-coloured rock which has been well broken into, in order apparently to see whether it was available for making “sets” or pitching stones, which form the staple trade of the adjoining district. Its surface is well rounded, as if by glaciation. The boss consists of at least three varieties of rock. Most noticeable is a coarse-grained hornblendic one of handsome appearance, the glistening cleavage planes of the brown hornblende standing out from the remaining green actinolitic portion. a second variety is a fine-grained greenstone, which is separated from the preceding by a hard definite line, so that it may be taken to be of the nature of a dyke intruded into it.