14 california academy of sciences. 



3. Pliocene. 



Deadman Island. — Overlying the Miocene shale of Deadman Island (see 

 diagram B, PL XXII) is a deposit of brown, clayey sandstone, varying in thick- 

 ness from twenty to forty-five feet. The distinct strata of this formation will be 

 described in detail. The surface of jointed shale on which the sandstone rests is 

 worn and uneven, but the contact conforms nearly to the dip of the shale, which is 

 between 20° and 30*^ northeast. Other evidence beside the worn condition of the 

 shales at the contact goes to show that the erosion took place while the shales formed 

 the sea bottom near the shore. Worm borings are common, and in one place a 

 pholas hole was found in the shale. The contact stratum, which is only about a foot 

 thick, is composed almost wholly of beach-worn pebbles of the Miocene shale, all 

 containing to some extent holes of worms and mollusks. It contains also many well 

 preserved shells and shell fragments. All of the fossils common in this layer are 

 found in the sandstone just above it, so its fauna will be taken up with that of the 

 overlying sandstone in a later part of this paper. 



This bottom Pliocene layer dips northeast at an angle of about 25°. Towards 

 the top of the formation the bedding planes become more nearly horizontal, those at 

 the top having a dip of only 8° or 10°. This could be accounted for in one of two 

 ways — either the lowest layer was deposited horizontally and then during the deposi- 

 tion of the subsequent layers there was a gradual uplift toward the southwest, or else 

 the lowest layer was deposited on a sloping bottom, and the general tendency of 

 sediments to settle in the lower portions of their basins and to form horizontal beds 

 gradually overcame the dip. 



About eight feet of fine, brownish yellow, clayey sand rests on the pebbly low- 

 est Pliocene stratum. In some of the places exposed to the action of the sea-water 

 this second Pliocene stratum consists of bluish gray clay, and in some places is filled 

 with well preserved fossils. In the southwest corner of the island the second layer 

 is a hard, fine, brown sandstone containing only a few fossils. 



Overlying the second stratum is a fine, dark brown sandstone about four feet 

 thick. The most fossiliferous places are hard, but porous. The fossils in these hard 

 places are well preserved, while those in the softer parts of the layer, which is lighter 

 colored, are poorly preserved and fragile. Pieces of the hard portions of the Plio- 

 cene stratum have broken off and have fallen among the fragments of shale along the 

 beach. This has caused some people to report the fossils found in this Pliocene bed 

 as occurring in the Miocene shale. On account of the great abundance of Tlnjasira 

 fCryptodon) bisecta in this stratum it has been given the local name of "Cryptodon 

 bed." Lucina acutilineata is also very common in the Cryptodon stratum. 



Above this very fossiliferous stratum is a bed of brown sandstone from twenty- 

 five to thirty-five feet in thickness, only slightly laminated, and varying somewhat in 

 structure and appearance in different parts of the layer. The lower part is uni- 

 formly soft, while toward the top the color is lighter and the rock much harder. 

 Only a few fossils, such as Lucina acutilineata and Fecten caurinus, have been found 

 in the upper brown sandstone stratum. The total thickness of the Pliocene beds at 

 Deadman Island is about forty-five feet. 



