WILLIS] GENERAL GEOLOGIC NOTES 57 



coast line and the special features thereol'; and finally as to the fossil 

 content. 



The Argentine coquina is composed, in tlie several places where 

 the writer observed it, of materials derived directly from the adjoining 

 strand. The sand, the pebbles, the larger stones, and the shells are 

 all identical, so far as they possess similar characteristics, with those 

 which make up the latest beach deposits. The sand is both white 

 and black, being composed of grains of quartz and of various dark 

 grains derived from volcanic rocks. The pebbles are in large part 

 those which have been styled "Patagonian," because they are the 

 same as those which form the Avidespread pebble deposit of the 

 Patagonian plateaus. They are brought along the shore by tlie pre- 

 vaihng northerly drift of the inshore currents. The shells and shell 

 fragments are those of thick-shelled bivalves wliich occur in limited 

 quantities on the beach and live in adjacent waters. Tlius, as 

 regards constitution, the coquina is identical with the beach. 



The physiogi'aphic relations of the coquina are definite. It occurs 

 only in immediate relation to the present coast within reach of the 

 waves or spray. It usually lies at the beach level. In one case, 

 however, namely, at the mouth of the Arroyo del Barco, to the 

 south of Mar del Plata, the coquina occurs up to a maximum alti- 

 tude of 9 meters above the beach; and again at the Boca del Moro, 

 north of Necochea, it was observed just above the modern beach, 

 lying in a slope continuous with the beach, but eroded by gullies. 

 At the Arroyo del Barco the coquina lies in an angle of the coast, at 

 the head of the Playa do Peralta, where it is open to the storm waves 

 of the Atlantic and at a point where the converging shores concen- 

 trate them and give them exceptional force. At the Boca del Moro 

 the occurrence of the coquina may be within the reach of an unusual 

 storm, but it is not unreasonable to postulate that the shore currents 

 have built out the sandy beach and have thus widened the strand 

 sufficiently to remove the zone of former wave action from that 

 of the present breakers. Another occurrence of this formation is 

 on the slope of Punta Porvenir, south of Mar del Plata. It there 

 consists of a mingling of sand and loess and extends from the low 

 bluff back of the beach to a distance of 110 meters from the shore, 

 that is to say, to the limit which may be reached by spray from the 

 storm waves breaking on the prominent point. It was in this deposit 

 that the Ameghinos found the carapace of a glyptodon associated 

 with hand- worked stones. Just north of Punta Piedras at Mar del 

 Plata the coquina occurs on the beach in the lee of the quartzite 

 point, where the waves have washed in the material which goes 

 to make up the conglomeratic and pebbly deposit. It also consists 

 largely of shells and is cemented by lime. A peculiar interest 

 attaches to it because it appears to underhe a portion of the bluff. 



