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 :-
Peterhead blue granite .
Aberdeen red granite .
Aberdeen blue granite .
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.