49 analyses (47 by spectrographical and 2 by classical methods) of Middle and Lower Ordovician Shales from the Oslo District are presented. ldentification and semiquantitative measurement of the major mineral constituents were achieved by X-ray diffractometer methods. A marked increase in the MgO content of the shales occurs at the boundary between the Lower and the Middle Ordovician and is related to a higher chlorite content in the latter. There is a possible coincidence between the upthrusting of the basic nappes (Lower Jotun Nappe) 200 km to the west and this marked change in geochemistry. A comparison of the Ordovician geochemistry of the Oslo Region with that of South Sweden suggests that stratigraphic variation in this area is more distinct than regional variations. The significance of this conclusion in relation to other parts of the Caledonides is discussed.