580 A. W. GRABAU LOWER ORDOVICIC FORMATIONS 



In Normandy the Cambric, when present, rests with an unconformity 

 on the folded and eroded "phyllades de Saint L6" of Algonkic age. In 

 the province of Orne the Gres Armoricain rests disconformably on the 

 Cambric, but in the southern part of the adjoining province of Manche 

 it rests unconformably on the Saint L6 phyllites, the Cambric, if deposited 

 here, evidently having been eroded before the deposition of the Armorican 

 sandstone. Moreover, the Armorican sandstone has a transgressive char- 

 acter and varies in thickness from 12 to a maximum of 500 meters. It 

 is followed disconformably by the Angers roofing slates (Schistes a Caly- 

 mene), with a thickness of 30 meters or Jess, which contain in their basal 

 part Didymograptus geminus and Calymene aragoi, and Calymene tris- 

 iani and Trinucleus bureaui in the upper part. Other species are: 

 Dalmanites phillipsi, Uraliclias ribeirei, Asaplius guettardi, Illcenus gi- 

 ganteus, Placoparia tourneminei, Cheirurus andegarus, ostracods, cepha- 

 lopods, pelecypods, and brachiopods, including Orthis budleighensis. 

 Near Caen (Calvados) the Angers slates are succeeded by the May sand- 

 stone (gres de May), which still contains in its lower part Calymene 

 trisiani and is otherwise rich in trilobites. 



In the valley of the Mayenne the pre-Cambric beds are succeeded, appar- 

 ently with a disconformity, by the Gourin pudding-stone, and this by the 

 Armorican sandstone, the Cambric being absent here. That the Cambric 

 formerly extended over much of this region is shown by its presence in 

 the basin of Laval, which lies between this region and southern Manche 

 (Mortain). where the Cambric is likewise absent, though to the east in 

 Orne it is again present. From this we may conclude that the epoch of 

 the Armorican grit was preceded by one of erosion, which removed the 

 Cambric formations from parts of this region. This is also shown by the 

 conglomeratic character of the base of the Armorican, where it rests on 

 the Cambric (see in Brittany). The Armorican sandstone and the suc- 

 ceeding Angers slates with Calymene trisiani are here much thinner, 

 the hiatus between them being greater than farther north. At Montigue, 

 on the southern flanks of the Laval basin, the Armorican sandstone is 

 reduced to a few thin, sandy beds with Lingula lesueuri. This rests 

 without the basal conglomerate on the pre-Cambric beds, apparently with 

 a disconformity, and is succeeded by the Angers series, somewhat slaty, 

 very black, and poor in fossils. This is followed by the Saint Genua in- 

 sur Tile sandstone with Orthis berthoisi var. erratica. 



These sections thus indicate a transgression of the sea from the north 

 over the old Armorican land-mass which included the central area of 

 France. After the deposition of the Armorican sandstone, the sea again 

 withdrew, to return in mid-Ordovjcic time, when the Angers argil lulites 



