2 MR GOODCHILD. 
in places; so that what was originally Opal may locally pass 
into crystalline Quartz, or into Amethyst. Excellent examples 
of these modifications of Opal are exhibited in the fine Heddle 
Collection of Scottish Agates presented to the Edinburgh 
Museum of Science and Art by Mr A. Thoms of St 
Andrews. 
The natural colowrs of agates are usually tertiary colours 
of some time, commonly slate-colour, or pearly-grey; less 
commonly café aw lait, pale umber or wood-brown. Rarely 
or never is a true agate originally bright yellow or red; and 
only in the case of some exceptional mixtures of Chalcedony 
and Green Earth is it green, as in the case of Heliotrope or 
Bloodstone. Scottish agates come chiefly from the andesitic 
lavas of the Caledonian Old Red, which was locally covered, 
unconformably, by the Upper Old Red Sandstone. From the 
lakes which occurred in Scotland during the time when 
this rock was being found, there penetrated downward water 
charged with unusually large percentages of iron in solution. 
These ferriferous solutions have percolated through the rocks 
and into the agates, staining them various shades of red, 
ranging from pale Indian Red to almost vermilion, but leaving 
them still translucent. Carnelian Agutes, such as occur in 
the Cheviots, and at Carslogie, near Cupar, in Fife, have 
attained their bright red coloration in this way; while in 
other districts, where the colouring process has been less 
regular, the agates are stained in a variety of other ways. It 
will be observed, however, that certain layers have invariably 
resisted the staining. Usually the iron takes the form of 
minute spheroids of ferric oxide, as already noted, but in 
some few cases ferric hydrate has been the colouring material, 
and the agate then shows ochreous stains. 
At some period late in the history of an agate, solutions 
of manganese may percolate into cracks within the agates, 
and there consolidate as the well-known dendrites (? Psilo- 
melane). All true Mochas have been formed in this way. 
Moss Agates are contemporaneous structures, as noted above. 
Exposure to the influence of surface-water tends to 
bleach agates and then to discharge the colouring matters 
due to iron. Sea-water affects them differently, and tends 
