lO 



NATURE 



[May 2, 1878 



however, does not mix up what is merely speculative 

 with well-established conclusions in such way as to 

 mislead his readers who may be entering upon the study 

 of astronomy— a failing of too many works issued at the 

 present day. 



As affording a thoroughly reliable foundation for more 

 advanced reading, Prof. Newcomb's "Popular Astro- 

 nomy" is deserving of strong recommendation. 



J. R. Hind 



SLATE AND SLATE QUARRYING 



A Treatise on Slate and Slate Quarrying, Scientific, 

 Practical, and Commercial. By D. C. Davies, F.G.S. 

 (London : Crosby Lockwood and Co., 1878.) 



AMONGST the manufacturing industries which, during 

 the last hundred years, have expanded into large 

 proportions is the production of roofing slate. Nor is it 

 difficult to account for this expansion. Building opera- 

 tions have in this period progressed both over town and 

 country, both in' the Old and New World with extraordinary 

 rapidity, while canals and railroads have facilitated the 

 transport of the roofing slates from their mountain sources 

 to all parts of the land, and ships traverse the seas with 

 cargoes of the same material to various countries. Before 

 the introduction of canals and railways into the British 

 Isles, the slates of Wales, Cumberland, Scotland, and Ire- 

 land were restricted to the immediate neighbourhood of 

 the quarries from which they were extracted, and buildings 

 in various parts of the country far removed from these 

 quarries, were supplied with roofing materials from other 

 sources. In many districts tiles of burnt clay formed the 

 only available material, while in others, flag-stones and 

 tile-stones from the Carboniferous, Triassic, or Oolitic 

 formations were extensively used. In the eastern and 

 central districts of England the tile-stones of Stonesfield, 

 near Oxford, those of the Cotteswold Hills, and of CoUy- 

 weston formed an available source of supply ; and it must 

 be admitted that their greyish colour and general appear- 

 ance harmonise well with the prevalent Gothic or Tudor 

 styles of architecture of those districts. To such an 

 extent is this"admitted that_ these tilestoncs (erroneously 

 called " slates ") are still largely used^ in the counties of 

 Northampton, Oxford,^ and Gloucester, even when the 

 Welsh slate might be obtained at an equal or less cost, 

 and, owing to their heaviness, the high-pitched roofs, 

 which are so ornamental, and add so much to the 

 appearance of buildings, became a necessity. Never- 

 theless, the Oolitic tilestones are inferior in strength, 

 lightness, and durability to the latter material, and are 

 only used where aesthetic considerations prevail over 

 those of economy. 



In no country of equal extent has the art of slate- 

 quarrying reached such proportions as in the British 

 Isles, and especially amongst the mountains of North 

 Wales, which is its principal seat ; and considering the 

 magnitude of the works carried on, the large number of 

 persons employed, and the enormous sums made and lost 

 in this branch of trade, it is somewhat strange that no 

 work specially devoted to the subject of slate quarrying 

 has appeared up to this time. We therefore welcome 



Mr. Davies' little treatise, in which will be found a large 

 amount of interesting and valuable information regarding 

 the slate industries of North Wales and other districts, 

 gathered from much experience and observation, and 

 placed before the public in a very readable form. 



Mr. Davies, being a geologist, treats his subject geolo- 

 gically, recounting the various formations of North Wales 

 in which the various " veins " or beds of the best slates 

 are to be found ; and giving numerous details, often 

 illustrated by sketches of the stratigraphical phenomena 

 which are encountered in the quarries. Few people have 

 any idea of the physical impediments which occur in such 

 places. What with dykes, veins, slips, the disappearance 

 of cleavage planes, the local change in texture and com 

 position of the slate itself, and other disturbances and 

 " troubles," it is only a comparatively small proportion of 

 the entire slate-bed which can, even in the best quarries, 

 be converted into marketable slates, of the larger sizes 

 and qualities, known as " Princesses," '' Duchesses," 

 and " Countesses." Hence it is, that while a band of 

 slate-rock, in a favourable position for carriage, and com- 

 paratively free from such impediments to its useful 

 application, is a source of profit, another band, which is 

 not so free from these].'accidents>f stratification, remains 

 a profitless, or ruinous possession. 



Commencing with th e oldest formation of North Wales, 

 the Lower Cambrian of Prof. Sedgwick, Mr. Davies 

 describes the eminently successful quarries opened in this 

 formation in the Pass of Llanberris, which have proved a 

 source of untold wealth to their fortunate owners. The 

 slates from this formation are generally purple or greenish 

 — locally becoming greyish — and are amongst the 

 smoothest and strongest in Wales. The succeeding 

 formations of the Upper Cambrian and Lower Silurian 

 are also productive of slates — generally of bluish and dark 

 tints — and are worked over the central portions of the 

 mountain region. The value of such beds of slate depends 

 chiefly upon the uniformity and fineness of the grain of the 

 slatej and the facility with which the rock splits along the 

 planes of cleavage — which, as all geologists are aware, are 

 independent of those of bedding. In reference to the origin 

 of the cleavage structure, we are glad to find that the author 

 adopts the " mechanical . theory," which ascribes the 

 structure to the enormous lateral pressure to which the 

 rocks have been subjected when undergoing contortion ; 

 but in enumerating the observers who have contributed 

 towards our knowledge on this subject, he has omitted 

 the name of the late Mr. Daniel Sharpe, whose remark- 

 able papers published in the Journal of the Geological 

 Society (vols. iii. and v.), clearly established the relation- 

 ship between cleavage and pressure, and the structural 

 alterations which have been brought about within the 

 mass of the slate-rock itself, as subsequently confirmed 

 and illustrated by Mr. Sorby, after the microscopical 

 examination of thin sections. 



The author's notices of slate-production in districts 

 other than those of North Wales are scanty, and consist 

 of extracts from other works. His book is, therefore, 

 mainly valuable for the information it affords regarding 

 the position, structure, and mode of working the bands of 

 slate in the lower palaeozoic formations of North Wales ; 

 and as such it will be found a useful guide-book. 



