THE TILL. 539 



fied deposit, and one needs only to examine a fresh exposure of it and see 

 how all its .parts are thrown together in confusion, without any assortiuo- 

 according- to the weight and size of the stones — here a laro-e bowlder 

 projecting, there many small ones grouped, and again over Ijroad surfaces 

 the dark-gray compacted clay occurring almost free from stones of con- 

 siderable size and lacking all signs either in the color or the grain of a 

 lamination or an assoi-traent into jjaralle! layers — one needs only to make 

 these (observations and then for comparison examine the clay banks or sand 

 and gravel beds so well exposed in the river banks, remembering that they 

 are instructive only in a somewhat fresh exposure, to be convinced that all 

 the characteristics of water action — the delicate sorting and arran<j-ing, like 

 with like, according- to size and weight — are here markedh' absent, and that 

 it is quite impossible to explain the bed as formed in this way. 



If one has reached this conclusion by carefully comparing the two 

 formations and has the opportunity to examine many sections of the drift 

 where it is a fine sandy clay, he will be almost startled to find isolated 

 patches which seem to show a true and delicate lamination — a series of fine, 

 horizontal, parallel fissures, a few millimeters apart, usually gently undu- 

 lating. At times the undulations of adjoining lines meet at equidistant 

 points like a flat-meshed net, or like the cleavage of lioridilende, so that the 

 cla}- is separated into a bundle of flat, sharp-edged blades. These lines 

 fade away, however, in all directions into the general formless mass, and 

 constitute not a lamination in the technical sense — a result of deposition 

 in water — but a pressure cleavage caused by the same force which had 

 compacted the whole stratuuT. The eflect of considerable jwessure in 

 producing cleavage, or a tendency to split at right angles to the direction 

 of the force applied, may be seen in a variety of instances, and its recog- 

 nition has thrown light upon important problems of geologv, such as 

 the delicate banding of glacier ice and the smooth splitting of roofing 

 slates. Gun-cotton jn-essed into cakes, or thick pasteboard calendered under 

 heavy pressure, niay be separated easily into thin layers, and even the 

 splitting of a common cracker or the flaking of pastry is a structm-e pro- 

 duced by the pressure of rolling out the dough and developed afterwards in 

 the baking. This stritcture was well seen in the waterworks ditch o})posite 

 Pluienix Row in Amherst, and in the canyon of Deei-field River through the 

 divide range, described in the first section of Chapter XV, p. 509. 



