Johannes Jakob SederholmBorn 20 Jul 1863; died 26 Jun 1934 at age 70.Jakob Johannes Sederholm was a Finnish geologist who pioneered the study of the Precambrian rocks of Finland. He was appointed geologist to the Geological Commission of Finland in 1888, where later he became its director (1893 to 1933). The Precambrian period ran from 3.96 billion to 570 million years ago. Precambrian rocks, called sheilds can only be found today in Africa, Europe, and North America. Not much of these rocks can be found, but the ones that are display evidence of having been altered by intense metamorphosism. The minerals that the sheilds are made of are generally granite, schist, or gneiss.