RELATIVE AGE OP DETROIT RIVER SERIES 75 



developed basal conglomerate in which pebbles of the Detroit River series are 

 mingled with sand and Onondaga fossils and which rests on an uneven surface 

 of Detroit River series. Certainly there can be no doubt but that at this place 

 the time interval between the fossiliferous Detroit River series and the over- 

 lying Onondaga was very long. 



The most important consideration, however, is the condition at Amherstburg, 

 since it is there that doubt has been expressed regarding the occurrence of 

 an extensive unconformity (disconformity) at the base of the Onondaga. At 

 Amherstburg the development of the basal conglomerate is weak; in fact, 

 various geologists to whom specimens from this horizon were submitted ex- 

 pressed grave doubt as to the real conglomeratic nature of the deposit, although 

 admitting the intermingling of the Anderdon and Onondaga limestones at the 

 contact. This basal layer of the Onondaga is composed of more or less 

 angular fragments of the compact drab Anderdon intermingled with the rather 

 porous brown magnesian Onondaga, and in which there is sometimes so much 

 sand that a thin layer might be called a real sandstone. In addition to the 

 unevenness (see plate 7, figure 1), the upper surface of the Auderdon is rough 

 and uneven. This roughness is attributable to differential weathering, since 

 recently weathered surfaces of the Anderdon show the same pitted surface, 

 although the rock is fresh at a small fraction of an inch below the surface. 

 In some of these irregularities considerable accumulations of sand occur and 

 fragments of the Onondaga often cling to these irregularities after the over- 

 lying layer has been removed. The Anderdon is affected by a system of joints 

 which do not pass upward into the Onondaga. Some of these joints have been 

 widened by solution to three or more inches before the Onondaga was de- 

 posited, and then into these cracks the Onondaga muds, intermingled with 

 sand, filtered as the sea advanced over the region (see plates 7, 8, 9). In 

 some cases this sand has penetrated several feet below the contact and may 

 now be found in cavities among the Anderdon fossils. The upper surface of 

 the Anderdon is often pretty well covered with rather large, low-spired gas- 

 tropods and somewhat similar cephalopods (see plate 9). These are almost 

 always of the brownish gray Onondaga limestone, which has led some one 

 to suggest that the underlying mud must have been soft when the Onondaga 

 sea advanced over it, and at which time these gastropods and cephalopods 

 were pressed into this soft mud. Unfortunately these fossils are poorly pre- 

 served and have not been satisfactorily determined. Grabau and Sherzer say 

 that the gastropod is probably Trochonema ovoides, 7 but they do not even 

 mention the cephalopod. Possibly it is the Trochoceras anderdonense which 

 they have described. The indications are that these gastropods and cepha- 

 lopods were already fossil when the Onondaga sea advanced over the region ; 

 in fact, they were molds of the exterior and interior of the shells into which 

 the Onondaga mud was pressed. Frequently it is possible to find casts of the 

 shelly portion made up of the Onondaga material, which contrasts strongly 

 with the other, while the internal mold of the Anderdon material is still 

 retained and forms a perfect core for the fossil. Many of the infillings of 

 these gastropod and cephalopod molds are in large part sand. One or two 

 somewhat similar fossils were found in the basal Onondaga above the contact, 

 but they were so poorly preserved that it was impossible to make out whether 



7 Mich. Geol. and Biol. Survey, Pub. 2, Geol. ser. 1, 1909, p. 44. 



