Sills refers to the concordant igneous bodies that are formed when hot magma intrudes and gets trapped between the relatively older sedimentary rock layers. This are oriented parallel to the ground surface.
D(i)kes refers to the discordant igneous bodies that are formed when the hot intruding magma rises up vertically towards the surface.
Both these structures ranges in thickness from about a few millimeter to a few meters.
While it is difficult to determine exactly how many Natives lived in North America before Columbus, estimates range from a low of 2.1 million to 7 million people to a high of 18 million.
