The Power-House by John Buchan.  
The Power-House is a novel by John Buchan, a thriller set in London, England. It was written in 1913, when it was serialised in Blackwood's Magazine, and it was published in book form in 1916. The narrator is the barrister and Tory MP Edward Leithen, who features in a number of Buchan's novels. The urban setting contrasts with that of its sequel, John Macnab, which is set in the Scottish Highlands.  
The Power-House of the title is an international anarchist organization led by a rich Englishman named Andrew Lumley. Its plan to destroy Western civilisation is thwarted by Leithen with the assistance of a burly Labour MP.  
"The dominant theme of Buchan's fiction is the fragility of civilisation," it has been said in the context of a discussion of The Power-House. What the critic Christopher Harvie calls "perhaps the most famous line in all Buchan" occurs during the first meeting between Leithen and Lumley, when the latter tells the former, "You think that a wall as solid as the earth separates civilisation from barbarism. I tell you the division is a thread, a sheet of glass" (Chapter 3). Harvie cites a comparable passage from the second volume of The Golden Bough, where Frazer speaks of "a solid layer of savagery beneath the surface of society," which, "unaffected by the superficial changes of religion and culture," is "a standing menace to civilisation. We seem to move on a thin crust which may at any time be rent by the subterranean forces slumbering beneath."