72 



SCIENCE 



[N. S. Vol. XXVII. No. 680 



meriting material being opaline silica. As 

 above mentioned, the clays are generally green 

 and drab, but in the vicinity of Thomasville 

 there are white clays containing as little as 

 tv70 per cent, of iron oxide. 



The grit or sandstone feature of the Alta- 

 maha, the feature which is most striking and 

 which was first studied, is typically exposed 

 along the Altamaha River. It consists of 

 gray or greenish aluminous sandstone more 

 or less mottled and stained by iron oxide. In 

 restricted localities pebbles are imbedded in 

 the sand and clay matrix, and cemented into 

 a conglomerate; but, except for the pebbles, 

 these beds do not appear different from the 

 typical Altamaha sandstone. The percentage 

 of clay in the indurated rock varies from five 

 to ten per cent, to such a high percentage that 

 the rock is an indurated clay rather than a 

 sandstone. The cementing material is an 

 opaline silica and the rock may be extremely 

 hard and even glassy and quartzitic in appear- 

 ance, but is generally, however, soft and fri- 

 able. The rock is strikingly similar in its 

 lithological aspect throughout widely separated 

 areas and is easily identified, although it is 

 entirely devoid of any fossils. Except along 

 the Altamaha River, only widely separated 

 outcrops occur. It never presents any great 

 thickness, jutting beds fifteen or twenty feet 

 thick being exposed over a few acres. Ex- 

 posures of grit are common throughout the 

 northern part of the Altamaha region, but are 

 not observed near the coast nor the. Florida 

 boundary line. It is believed that these iso- 

 lated exposures are local indurations only and 

 not parts of a continuous sandstone bed. 



While in the foregoing the sand, clay and 

 sandstoiie are described separately, they do 

 not form stratigraphical units. Sand, clay 

 and sandstone may be seen in the same ver- 

 tical section, in which the clay may be re- 

 placed by sand, and sand and gritty clay may 

 be seen gradually changed from non-indurated 

 to indurated rock, from soft sand and clay to 

 typical Altamaha grit. 



The thickness of the formation can be as- 

 certained only from data from deep well 

 borings. The Altamaha in Georgia attains a 



known thickness of 350 feet and probably 

 reaches 500 feet. From this maximum thick- 

 ness, which occurs in the counties of Emanuel, 

 Tatnall, Toombs, Coffee and Tift, it is at- 

 tenuated both northward and southward. The 

 thickness in the southwest part of the state 

 can hardly be more than 200 feet as a maxi- 

 mum, and on the Atlantic coast probably does 

 not exceed 100 feet. As a whole, the study 

 shows evidence of a rapid deposition in a 

 shallow basin-like sea. There is a notable 

 absence of any calcareous layers or nodules, 

 chemical analyses, even of both the clays and 

 the sandstone, showing only a trace of lime 

 or none at all. The material composing the 

 Altamaha was largely derived from the meta- 

 morphic and igneous rocks of the Piedmont 

 region. 



The Altamaha has been observed overlap- 

 ping and overlying Eocene, Lower Oligocene, 

 Upper Oligocene, Miocene and questionably 

 Pliocene strata and is to some extent a super- 

 ficial deposit. 



In Burke and Washington counties, in the 

 northeast part of the coastal plain, the Alta- 

 maha is observed overlapping red sand and 

 gravel belonging to the Claiborne (Eocene) 

 group. The red sand of the Claiborne con- 

 tains thin quartzitic layers with Eocene fos- 

 sils, and can hardly be mistaken for the Alta- 

 maha, although the two may in places have 

 similar texture and color. 



It overlaps Lower Oligocene rocks (Vicks- 

 burg group) in the counties of Pulaski, Dooly 

 and along the east side of Flint River down 

 to Camilla. Near the contact with the Vicks- 

 burg group it is frequently found to contain 

 flint fragments, which are evidently derived 

 from the Vicksburg. 



At a number of points in Decatur, Grady 

 and Thomas counties, the Altamaha overlies 

 beds of Upper Oligocene age, seemingly with- 

 out unconformity. On the Monticello road, 

 four miles southwest of Boston, red and brown 

 sands of the Altamaha pass by a gradual 

 transition into clay and sand containing frag- 

 ments of oysters and the coral, siderastrea, 

 which identify the beds as Upper Oligocene. 

 This might be taken as evidence of identity 



