When this disturbance took place the granite juttings did not rise above the surrounding landscape as they now do. In many cases they did not even reach the surface of the shale beds, but ceased their flow and hardened short of the crust of the earth, as it was then to be found. When, however, the region was domed, many millions of years later, the subsequent weathering of the huge blister did not attack these granite formations with anywhere near the vigor with which the softer sandstones and limestones were eroded. What actually occurred, then, was a peeling away of the softer rocks, leaving the granite formations near their original sizes, but at last above the ground level in the form of peaks, needles, and spires. But we have gotten ahead of our story. Following the Algonkian period, when the molten matter was injected into the layers of shale, there came what is known as the Cambrian period. The Cambrian occupied the first 80,000,000 years of the Paleozoic era, which in itself covered the entire period from 510,000,000 to 180,000,000 years ago. During the Cambrian period the land subsided again, perhaps because of the weight of the uplifted sedimentary formations. During this subsidence the waters once again covered vast portions of North America, and additional muds and slimes were deposited on the bottom. It was at this period that life first appeared on the earth, in the form of simple marine organisms which have left fossil remains. These deposits made in the Cambrian period can be seen in outcroppings all through the region, although they are most notably found in the area about Deadwood. Because of their structure they indicate to the geologist that the shoreline of the ancient Cambrian sea was near at hand, and also that this covering of water was by no means as extensive or as deep as the earlier Archean sea. The deposits of sand and mud, which were eventually pressed into stone, occasionally reach a depth of as much as five hundred feet, although they were laid down extremely slowly, as eddying mud is laid at the bottom of a pond. In the locality of Deadwood they contained a rich infiltration of gold, and the entire conglomeration was thoroughly intermixed with a vast outcropping of much older rock—this effect undoubtedly having taken place later, during the great continental uplift, when the final doming occurred. THE AGES OF EARTH MILLIONS OF YEARS AGO (Pre-Cambrian Existence back to 3½ Billions of years)