OCCURRENCE OF A CHROMITE-BEARING. ROCK IN BASALT. 403 
Wales. The last mentioned rock, which contains chromite, occurs 
in the form of irregularly rounded blocks, the rounded character 
of which is probably not due to the mechanical action of water, 
but rather to a partial fusion of the rock in the magma of the 
originally molten basalt, which by corroding the edges and angles 
more than the other portions would rapidly convert angular 
fragments into rounded, just as mineral splinters become rounded 
buring the process of fusion in a borax head before the blowpipe. 
These blocks vary in diameter from a few inches up to about 
twenty inches. As they decompose less readily than the basalt 
they weather out from it, and can be readily separated from the 
matrix, when the latter is much decomposed, but less decomposed 
portions of the matrix are found to adhere very tightly to the 
blocks, and in such cases small fragments of the partially fused 
blocks appear to be present in the basalt near its contact with 
the enclosed blocks. 
Macroscopic Characters. 
At first sight there appear to be two distinct varieties of chrome- 
bearing rock present, the one a hard dense grey rock, like a very 
fine grained granite or felstone, which on freshly broken surfaces 
shows small particles of a jet-black mineral, with minute crystals 
of pyrites, and what appears to be small greenish stains, the other 
a greenish-grey rock, showing on freshly fractured surfaces patches 
of a green mineral, at first presumed to be malachite, intermixed 
with grey to brownish-grey material. A detailed examination 
however of these two varieties of rock shows that they probably 
belong to one and the same type, the difference in their general 
appearance being due partly to the relative amount of decompo- 
sition, which they have respectively undergone, and partly to the 
comparative variety or abundance of the mineral which has yielded 
the greenish decomposition products, and which was probably a 
chrome-bearing diallage, similar to that about to be described. 
One block of the chromite-bearing diallage and felspar with 
chromite passes gradually at the periphery into an external zone 
about three inches in thickness composed of the greenish-brown 
