Revieics — GrecnwcU 8f Elsden — Roads. 77 



Arranging their subject under three heads — (1) Materials, (2) Con- 

 struction, and (3) Maintenance — the authors in the first part deal with 

 the selection of road material, and point out the various kinds of 

 rock that have been used, and their geological distribution. Figures 

 of micro-sections are given to make clear the mineral structure of 

 various kinds of stone ; while their texture and absorptive power 

 and the joints in the rocks are duly explained. Special tables are 

 given of the principal constituents of some igneous rocks used as 

 road stone. 



The value of particular stones as paving setts is pointed out, and 

 it is observed that some igneous rocks wear to a smooth and slippery- 

 surface. Following the igneous rocks, the sandstones, limestones, 

 flint and chert, and gravel are successively dealt with. There are also 

 chapters on artificial materials, including slag, which is extensively 

 used in some neighbourhoods, though it yields an ofi'ensive odour and 

 is unsuitable for residential areas. Among paving materials, wood 

 and asphalt ai'e included. 



Tables showing results of various tests are given in reference to 

 some of the principal rocks, and in the ' percussion test ' we note 

 that the greenstone of Penzance withstood the greatest number of 

 blows. This is evidently the well-known Penlee stone, which is 

 elsewhere referred to under that name, and also as Newlyn stone. 

 After considering various tests of strength it is well observed that 

 the test of experience is the best ; but this is not always forthcoming. 



In the second part of their work the authors give a brief historical 

 account of roads from Eoman and mediaeval times to the days of 

 Macadam and Telford. A knowledge of geology is of essential 

 service in the laying out and construction of roads. Cyclists in 

 particular are shrewd observers of roads, and with a knowledge of 

 geology they may, as Mr. F. J. Bennett has pointed out, learn 

 much from the gradients, the springs, and the character of the 

 surface formed by the road-metal.^ The very diversity is interesting, 

 and we should not wish to see Telford's rule adopted that " It is 

 absolutely necessary to remove trees from the sides of roads, and 

 to keep the fences under five feet in height." 



The authors give numerous illustrations, some geological, to 

 aid in their explanations of road-making. Indeed, in this and 

 other respects the accumulated knowledge of the engineer and the 

 practical geologist is happily combined in this volume. Thus the 

 work, though professedly of an elementary nature, should prove 

 most useful to local road-surveyors and to others called upon to 

 find or advise in selecting road material. 



A fuller index than the authors have given would be desirable 

 in a new edition ; thus. Crushing strength, Cobbles, Mountain 

 Limestone, Oolite, Forest Marble, Greywethers, and a few 

 other subjects referred to in the text, should have been found. 

 Mention is made of certain tests of rocks carried out in Maryland, 

 and the reference might have been given to the third volume recently 



^ "Influence of Geology on the Early Settlements and Roads" : Proc. Geol. 

 Assoc, Yol. X, p. 377. 



