RELATIONSHIPS OF THE OAKS 171 



oak of to-day. Again, 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. Rolur, 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 Q. tindoria 

 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. 8ul>er. 



Q. infectoria yields the chief galls of commerce,. 

 They are caused by the punctures of Cynips gallce tinc- 

 tvricB, 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. 



