242 THE OLD RED SANDSTONE. 
brown ; and on this ground the fossil is spread out in strongly 
contrasted masses of white and dark red, of blue, and of pur- 
ple. Where the exuviz lie thickest, the white appears tinged 
with delicate blue —the bone is but little changed. Where 
they are spread out more thinly, the iron has pervaded them, 
and the purple and deep red prevail. Thus the same ich- 
thyolite presents, in some specimens, a body of white and 
plum-blue attached to fins of deep red, and with detached 
scales of red and of purple lying scattered around it. I need 
hardly add, however, that all this variety of coloring is, like 
the unvaried black of the Cromarty specimens, the. result, 
merely, of a curious chemistry. 
