428 Tarr — Origin of the Chert in the Bitrlington Limestone. 



cal weathering and transported to the sea by the streams as 

 colloidal silica. Areas of low-lying lands, especially peneplaned 

 areas, where chemical denudation predominates over mechani- 

 cal, would be especially favorable for furnishing increased 

 quantities of silica. Wide areas of igneous and metamorphic 

 rocks would also be very favorable for furnishing colloidal 

 silica, as large amounts of silica are liberated by the decay of 

 such rocks. Present streams are transporting large amounts of 

 silica to the sea, and under such conditions as peneplanation, 

 especially on areas of igneous rocks, the amount of silica would 

 be greatly increased. The theory advanced, however, is not 

 dependent upon a peneplaned area as the source of the silica. 

 The main idea is that silica was precipitated directly on the 

 sea bottom after it had been brought to the sea by the streams. 



The colloidal silica is believed to have been precipitated in 

 the sea by the action of the alkalic salts in the sea water after it 

 had undergone considerable dispersion, and a certain amount of 

 concentration. Since it is the tendency of all colloids to aggre- 

 gate into globular masses, a tendency which is very powerful 

 in silica, it is believed that the silica thus precipitated on the 

 sea bottom would tend to assume more or less globular to ellip- 

 tical forms. Upon burial these forms would become compressed 

 into their elliptical or lenticular shapes by the weight of the 

 accumulating sediments. Fossils falling into this soft colloidal 

 mass would be perfectly preserved. 



The various physical features of the chert, its form, position 

 in the beds, and microscopic characters, are well explained bj 

 the theory outlined above. 



Attention should be called to the many points of similarity 

 of Prestwich's theory, as given in his Textbook of Geology, to 

 the one given above, although the writer's theory was devel- 

 oped before seeing that of Prestwich. The points of differ- 

 ence lie in the mode of precipitation, methods of aggregation^ 

 and the discussion of the various features of the chert. This 

 paper develops the discussion as to the source of the silica and 

 its transportation. 



B. EVIDENCE FOR THE THEORY. 



1. Source of the Silica. 



{a.) Carried to the sea hy streams. — Though some of the 

 silica in the sea water may have been derived from the break- 

 ing down of silicates deposited upon the sea floor, it is proba- 

 ble that such a source would supply only a small quantity of 

 the silica actually present. J. Murray and P. Irvine^ state 



* Clarke, F. W., Bull., U. S. G. S., 491, p. 110. 



