Mr. Walker exposed the undermentioned descriptions of granite and whinstone to very heavy waggon traffic for seventeen months, and found their vertical wear to be as follows :-

Inch.

Guernsey granite

•060

Herm ,,

•075

Baltic whinstone

•082

Peterhead blue granite .

•131

Inch.

Heytor granite

•141

Aberdeen red granite .

•159

Dartmoor granite

•207

Aberdeen blue granite .

•225

Mr. Newton's experiments on the flags used in Liverpool showed Kilrush flags to be most durable, Caithness flags next. The flags found to be least durable were those from Llangollen and Yorkshire.1

1 D. Kinnear Clarke On Roads and Streets.