“But I am constrained to go one step farther in this hypothetical picture of structural defectiveness. To return to our wall of brick. It can be made up of bricks laid upon each other in consecutive tiers; it can be made up of tilted tiers of bricks, bricks laid on each other, but inclined to a horizontal plane, and finally it is conceivable that the bricks may be so arranged as to be inverted in their relations to the horizontal plane. The diagrams make clear these contrasted positions. “Now of all these types of structures the last obviously best meets the requirements of a type which will prove the least susceptible to dislocation. I think that can be apprehended almost without explanation. A moment’s reflexion will make it conspicuous. “The bricks tilted up in inclined tiers or beds, upon disturbance, if the cohesion between them is seriously impaired, tend to fall away from each other, and gravity increases the effects of the initial displacement. If the bricks lie flat they do not fall apart, upon the cessation of any push or upheaval, but remain disordered, falling back into some _quasi_-position of rest. If the bricks are inverted and form in section a series of lines converging to the base of the wall, their disarrangement is largely rectified by their own gravity, bringing them back into their first positions. “In Geology strata overlying each other, in succession, as the bricks do when on their flat faces are called _conformable_, if they succeed, one over the other, with the edges or summits of the lower, abutting against the horizontal surfaces of the next, as do the bricks when they are placed in flat and vertical positions, in alternating strips, that is _unconformability_. “If the strata are usually horizontal like the evenly piled series of bricks, they are called _undisturbed_; if inclined against each other, they are _inclined_, and they may make _monoclinals_, having one slope, or _anti-clinals_ when they lean up against each other like the opposite sides of a peaked roof; _or synclinal_ when inclined towards each other in an inverted position like the same roof overturned, with its ridge pole on the ground, and its inclined sides lifted into the air, or like the bricks in the last pattern of structure described. “When we carry these similes into nature, we have all kinds of rocks, and we have them in mountains, in planes, and all the familiar configuration of the earth’s surface.