Showing portion of a street in San Francisco after the terrible earthquake of 1906. The resemblance of the break in the ground to the appearance of a stationary wave should be noted.] What happens when it gives way? To answer this question we had better carry our minds to the example of the pill-box lid. If the top of the box were of very brittle material, like pottery or glass, then after the breakage we know that most likely one of the broken pieces would be a little higher than the other--would perhaps overlap it. That is what we often see when examining the geological earth strata. One stratum, instead of lying evenly with another where a crack has occurred, rests a little above it or below it. This inequality or unevenness geologists call a fault. Now we can easily see that whenever, and by whatever causes, a fault is produced, there is probably at the same time an earthquake. The fault cannot be produced without a great and shaking disturbance. Mr. John Milne, the most distinguished of British authorities on earthquakes, says that all large earthquakes originate from the formation or extension of these "faults" or great cracks in the strata. The occurrences of these "faults" are most frequent when the process of mountain building has been going on over the earth. In other words, if we imagine a period in which some great continental area of the world's surface was slowly rising above the oceans, then at that time we should expect that there would have been great "faults" occurring in the strata, and great earthquakes following them. To quote his words: "If therefore we wish to know when earthquakes were greatest and most frequent, we must turn to those periods in geological history when mountain ranges were built, when volcanic activity was pronounced, and when great 'faults' were made. The first of these periods would be coincident with the creation of the Ural Mountains, the Grampians, and other mountain ranges. This took place in the earliest geologic times. Another period of mountain formation was when the Himalayas and the Alps were slowly but intermittently brought into existence. In both these periods volcanic activity was pronounced, and beds of coal were formed. When the crust of the earth was crumbling, mountains grew spasmodically--(they sprang up, as it were, from out of the giant forces which we have described earlier in this book)--'faults' gave rise to earthquakes, volcanic forces found their vents, and conditions existed which gave rise to the accumulation of materials to form coal."