The Ruskin Library Transcript T7B here reads:
By solid lintels from pillar to pillar: a By sloping blocks or beams from pillar to pillar, b. By wedge lintels c or by arches d.
The changes it makes to the punctuation of the manuscript misrepresent what Ruskin wrote and obscure the meaning. The letters refer to the opposite page Notebook M2 p.7backL in which ‘a’ shows solid lintels from pillar to pillar; ‘b’ shows sloping beam from pillar to pillar; ‘c’ shows wedge lintels; ‘d’ shows arches.
[Version 0.05: May 2008]