_lm_, limestone; _sh_, shale; _r_, _r´_, and _r´´_, river silts filling eroded valleys in the limestone. The upper surface of the limestone is evidently a land surface developed by erosion. The valleys which trench it are narrow and steep-sided; hence the land surface had not reached maturity. The sands and muds, now hardened to firm rock, which fill these valleys, _r_, _r´_, and _r´´_, contain no relics of the sea, but Instead the remains of land animals and plants. They are river deposits, and we may infer that owing to a subsidence the young rivers ceased to degrade their channels and slowly filled their gorges with sands and silts. The overlying shale records a further depression which brought the lanes below the level of the sea. A section similar to this is to be seen in the coal mines of Bernissant, Belgium, where a gorge twice as deep as that of Niagara was discovered within whose ancient river deposits were found entombed the skeletons of more than a score of the huge reptiles characteristic of the age when the gorge was cut and filled] [Illustration: Fig. 206. Unconformity showing Buried Mountains, Scotland _gn_, ancient crystalline rocks; _ss_, marine sandstones. The surface _bb_ of the ancient crystalline rocks is mountainous, with peaks rising to a height of as much as three thousand feet. It is one of the most ancient land surfaces on the planet and is covered unconformably with pre-Cambrian sandstones thousands of feet in thickness, in which the Torridonian Mountains of Scotland have been carved. What has been the history of the region since the mountainous surface _bb_ was produced by erosion?] =Unconformities in the Colorado Canyon, Arizona.= How geological history may be read in unconformities is further illustrated in Figures 207 and 208>. The dark crystalline rocks _a_ at the bottom of the canyon are among the most ancient known, and are overlain unconformably by a mass of tilted coarse marine sandstones _b_, whose total thickness is not seen in the diagram and measures twelve thousand feet perpendicularly to the dip. Both _a_ and _b_ rise to a common level _nn´_ and upon them rest the horizontal sea-laid strata _c_, in which the upper portion of the canyon has been cut. [Illustration: Fig. 207. Diagram of Wall of the Colorado Canyon, Arizona, showing Unconformities]