58 



HARDWICKE'S SCIENCE-GOSSIP. 



skilfully built at the first will give the farmer no end 

 of trouble, and afford a freaksome group of children or 

 a mischief-working lounger plenty of amusement as 

 they push stone after stone from its place into 1 the 

 field below. How then can the stones be secured in 

 position so that weather and other agents may do 

 the dyke as little damage as possible ? Either some 

 stones may be placed on edge as a coping, or a cap 

 of mud may be placed on the top of the wall, and 

 beaten down into a compact tapering shield. 



In the case of the dyke new in our minds, the 

 latter plan has been adopted. It may be objected, 

 however, that to use any ordinary mud or wet mould 

 for this purpose would be to call in a very unreliable 

 material. Exactly so ; and the rule is, that when 

 such material only is available, turf is added, on 

 account of the tenacity of the grass roots and fibres ; 

 and thus, while the wall is often bare and dry, the 

 cap is covered with verdant grass or a variety of 

 flowering and cryptogamous plants. Nature, how- 

 ever, seems to be always making everything ready for 

 our hand, and it so happens that in many places where 

 the boulders necessary for forming dykes are plenti- 

 ful, a special kind of mud, called clay, is also found 

 in the neighbourhood, which is admirably adapted 

 for the purposes of coping the wall. Just as coal, 

 iron, and fire-clay are often found together, or in 

 near proximity, so boulders for building, and clay for 

 coping attend each other. Has the one anything to 

 do with the other then — did the same agency which 

 placed the boulders where they are ready for the 

 dyker also place the clay in position for his use as 

 well ? Undoubtedly ! And here it is that, on this 

 dull day in mid -winter, we are able to find something 

 to instruct and interest us. We observe that this 

 wall is capped with clay, which, having been 

 properly beaten into shape, has hardened and formed 

 an enduring cap, on which the rains and snows may 

 fall for many winters before the tenacious mud will 

 be worn away. We are thus led to ask : What is 

 this clay, and whence came it ? It is very different 

 from the mould in the adjoining field which has been 

 produced in situ (either in part or wholly) by the agency 

 of rain and frost, by the burrowings of earthworms, and 

 by a series of other changes on which I need not dwell.* 

 It is also a very different thing from the clay which ages 

 ago formed those slates which are quarried in other 

 parts of this county of Cumberland (see Fragments of 

 Science, by Tyndall, explaining what slates are) ; 

 and equally different from those other kinds of mud 

 or clay which we find in various parts of the country 

 and in various geological formations (as London clay, 

 lias, firebrick-clay, pipe-clay, terra-cotta clay, not to 

 mention others). Like them, it is mud ; like them, 

 it consists of the finely-ground portions of earlier 

 formations ; like them, it has been subjected to the 



* Those who wish to pursue this subject further should con- 

 sult Darwin's "Formation of Vegetable Mould," and Charles 

 Kingsley's "Town Geology," chap. i. 



action of water ; but, unlike most other clays, this 

 particular kind has been associated with a force 

 which has been at work, not in tropical seas, rivers, 

 or estuaries, nor in temperate climes, but among the 

 ice-bound regions of the north. What if we say that 

 these boulders and that clay are really portions of the 

 same rocks, and assert that they came from the same 

 formations or the same mountain heights ? True, the 

 clay seems tolerably homogeneous, while the boulders, 

 as we shall presently see, are heterogeneous ; but then,. 

 if the clay be analysed, it will be found to contain 

 nothing which cannot be found in the boulders as- 

 well ; and we are perfectly well aware that the only 

 real difference between the clay and the boulders is a 

 difference of size. The clay has been rubbed down 

 into a powder, and deposited while wet in such a 

 way as to form a bed, while the larger masses have 

 so far escaped the rolling and tumbling necessary to- 

 reduce them to an impalpable dust. The clay we 

 are examining has received the very appropriate 

 name of "Boulder-clay," on account of the presence 

 in it of these larger masses or boulders ; and geologists- 

 have now settled for ever the question of its origin. 

 So much has been written by various authors, from 

 Professor Geikie (The Great Ice Age) downwards, 

 that no one need now be told how the boulder-clay 

 came into existence.* So far as I am aware, the 

 boulder-clay is not found south of the metropolis, 

 and even where it does occur it varies greatly in 

 colour and character. It abounds in Cumberland, 

 and has been the subject of careful investigation on 

 the part of a few devoted students, whose conclusions 

 have been made public in " The Transactions of the 

 Cumberland and Westmoreland Association" (particu- 

 larly 1877 and 1887). 



Mr. Goodchild especially has done good service in 

 this direction by the publication of his paper on Ice- 

 work in Edenside. When the mighty glaciers of 

 olden time came sliding down the mountain valleys 

 of Northern Britain they brought with them the 

 materials which they had torn from the hill- sides and 

 mountain-peaks ; and while much of this was ground 

 to powder, and presently emerged from the bottom 

 of the glacier (in the stream which resulted from the 

 melting ice) to be carried into the sea, the boulders, 

 marked and striated, retained their position in the 

 ice-mass till the ocean-waters caused the cold grip to 

 be unloosened, whereupon they dropped into the clay 

 which had already been deposited by the stream ; 

 and thus boulders and clay became for ever after 

 associated with each other and with the glacial 

 epoch. 



Simple as this episode now appears, and interesting 

 as the mud -cap upon our roadside dyke becomes by 

 the light thus thrown upon it, the boulders of which 



* We may, however, refer our readers to "Town Geology," 

 chap, ii., "Elements of Geology" (Lyell), chap, xi., "Reports 

 of British Association," 1887 and other years, " Climate and 

 Time," by Dr. Croll, and other works. 



