RELATIONSHIPS OF THE OAKS Vi 
oak of to-day. Avain, fossil leaves from Italy, found in 
diluvial deposits, are so like those of certain Californian 
oaks now existing that a common origin is also suggested, 
and similar leaves have been discovered in Tertiary de- 
posits in North-west America. If all the evidence is 
put together we may conclude with Asa Gray that ‘the 
probable genealogy of Q. Robwr, traceable in Europe up 
to the commencement of the present epoch, looks east- 
ward and far into the past on far distant shores.’ 
Many of the oaks yield products which are made use 
of in the arts, apart from their timber, the most valuable 
of which comes from our European oak, the white oaks 
of North America, and one or two Himalayan species. 
In several countries oaks are grown for the sake of the 
bark, cups, &c., as a tanning material, and these even 
form important articles of export. Quercitron, a yellow 
dye and tanning material, is obtained from (). tinctoria 
in North America. 
Cork, as used for bottling and other purposes, is 
obtained in Spain, the South of France, and in Algiers, 
from the thick periderm of Q. Suber. 
Q. infectoria yields the chief galls of commerce. 
They are caused by the punctures of Cynips galle tinc- 
toric, and are used for making ink and for dyeing. In 
these and similar galls the value depends on the pre- 
sence of relatively large quantities of tannic and gallic 
acids which they contain. 
