Complex sentence: He studied the geysers of Iceland, and wrote also on the classification of some of the eruptive rocks; but his main work consisted in the systematic examination of the crystals of numerous minerals, in researches on their optical properties and on the subject of light polarization.
Split into simpler sentences: He studied the geysers of Iceland, and wrote also on the classification of some of the eruptive rocks. His main work consisted in the systematic examination of the crystals of numerous minerals, in researches on their optical properties and on the subject of light polarization.