306 University of California Publications in Geology t VoL - H 



some diatom frustules and sponge spicules. The siliceous earths are 

 very soft, low in specific gravity, and poorly consolidated. Generally 

 they are white in color at the surface, but below the surface they are 

 often yellow, drab or pink. The second member contains less than 

 one per cent of calcareous material and may contain as much as 

 seventy-seven per cent of organic silica. The rock is seen under the 

 microscope to be a mass of siliceous remains, the finer portion of which 

 consists of finely broken organisms with considerable argillaceous 

 matter. In the purest siliceous earths the tests are closely packed in 

 a matrix of siliceous fragments. In the more impure varieties the 

 argillaceous matter may make up half the rock. The most siliceous 

 of these rocks are permeated with silica in a colloidal state, probably 

 derived from the siliceous organisms. 



3. The third member which lies above the siliceous earths and 

 grades into them, is a calcareous earth, forty-five feet thick, contain- 

 ing layers of pumiceous sand. Calcium carbonate, largely in fora- 

 miniferal shells, varies from forty to sixty per cent. 



Both the upper and lower foraminiferal marls contain from one 

 to twenty-five per cent of colloid silica, which appears to be diffused 

 through the mass of the rock. Siliceous organisms rarely occur in 

 these rocks, though the silica which they contain was probably once 

 in the form of organic remains. In the transition beds, which contain 

 both calcareous and siliceous remains, the radiolarian skeletons are 

 poorly preserved. 



In both the siliceous and calcareo-siliceous earths, there occur 

 cherty nodules. These consist largely of siliceous skeletons, which are 

 cemented by amorphous silica. 



4. Above the upper calcareous beds there is a mass of very fine 

 argillaceous earth, which has a thickness of twenty-five feet, and is 

 of various colors, red, pink, yellow, white or mottled. It is regarded 

 as being the equivalent of the Red Clay of modern abyssal ocean 

 depths. It contains only a trace of calcium carbonate in an occa- 

 sional foraminiferal shell and consists of fine argillaceous material 

 with a few radiolarian skeletons. The argillite is exceedingly fine 

 grained and has a peculiar greasy feel. The mineral fragments which 

 it contains are angular, as are those of the present red clay. Feldspar 

 is rather common in these fragments, while quartz is rare. 



Harrison and Jukes-Brown 41 made a series of analyses of modern 

 Bed Clay for comparison with those of the variegated red and yellow 



41 Harrison and Jukes-Brown, Notes on the Chemical Composition of some 

 Oceanic Deposits, Quar. Jour. Geol. Soc, vol. 51, p. 313, 1895. 



