Our big job was to map the laccoliths near Deadwood. A laccolith, or rock cistern, is a lava body which in very ancient times squirted into the cracks of the strata. The lava had penetrated between the strata of the northern cover of the Black Hills, swelled to lenses between the strata; and, particularly, it selected and penetrated the soft shale beds which grow thicker and more numerous upward among the formations. Thus after erosion of the present landscape, both large and small lava lenses were revealed as resistant hills, the largest toward the bottom of the pile of strata and the smallest and steepest toward the top in thick, black ancient mud deposits. Mostly, the laccoliths were injections of volcanic fluid up a crack, which met a hard bed and bent to squeeze the paste or lava into a soft layer. The result was an underground lava flow which ruptured the beds. Apparently the first rush brought up fragments of the rocks below. This fragmentary stuff of mud and gravel was overridden by the lava, until the latter penetrated horizontally a mile or two between strata, arched the layers above, and solidified at the Devil’s Tower with vertical columns like the Giant’s Causeway in Ireland. This group of subterranean volcanic eruptions between strata probably came under sea bottom at the same time that the Yellowstone upland began its open-air outpourings farther west. But in the Black Hills there is no sign the laccolith lavas ever broke up to the top country. The Black Hills, like the Rocky Mountains, were a long time rising in waves of action, whereas the lava intrusion was a relatively short episode of one of the latest of these spasms. However, that episode entails a long story of numerous injections. It takes us down into crust and along through the millennia. Always think in millions of years. It is wise also to think in millions of miles and to remember that the sun and the Milky Way are parts of the same system as the earth. And remember that a ledge or a boulder doesn’t worry about living 20 million or 100 million years. A skull is a boulder. That old brontotherium rhinoceros with a forked horn, standing eight feet high and fifteen feet long, lived in the upper Oligocene, when clay and volcanic ash were being deposited in the Bad Lands of South Dakota. Probably vast flood plains of rivers were his habitat, swamp reeds and leaves were his food, and floods washed his bones and buried his skull where we find them today. The country of open glades was probably like the safari land of central Africa.