oaks now existing that a common origin is also suggested, and similar leaves have been discovered in Tertiary deposits in Northwest America. If all the evidence is put together, we may conclude with Asa Gray that " the probable genealogy of Q. Robur, traceable in Europe up to the commencement of the present epoch, looks eastward 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, etc., as a tanning material, and these even form important articles of export. Quer-citron, a yellow dye and tanning material, is obtained from Q. 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 gallæ tinctoriæ, and are used for making ink and for dyeing. In these and similar galls the value depends on the presence of relatively large quantities of tannic and gallic acids which they contain.