418 J. BARRELL RECOGXITIOX OF ANCIEXT DELTA DEPOSITS 



divided into several cases. First are those variegated beds, consisting of 

 clavs intercalated with silts or argillaceous and ferruginous sands, in 

 which the clays show an absence of ferric oxide as contrasted to its pres- 

 ence in the other strata. Such a formation, now in process of origina- 

 tion, has been studied by Huntington in the delta of the Helmund and 

 the lake of Seistan, of eastern Persia. The latter is a shallow and vari- 

 able water body in the center of the waste-filled basin. The margins of 

 the lake support a dense growth of reeds and the bottom deposits consist 

 of a fine greenish or white clay. Over the subaerial plain the deposits 

 range from a silt near the head of the delta to a more cla^^ey nature near 

 the lake. The upper parts of the delta are well drained and the soil well 

 aerated. Over these areas light brown is the dominating color. The 

 flat swampy clay land near the lake may have been partly deposited as 

 lake beds and are now through considerable periods of time overflowed 

 or at least saturated with water. In these places the browns fade out and 

 light-colored soils with black areas j^revail.^^ The relation of the present 

 surface deposits to the past accumulations are well shown where part of 

 the lake bottom has been uplifted by recent volcanic action and erosion 

 has exposed the sequence of beds. From these Huntington has inter- 

 preted the Pleistocene history. The significant feature from the present 

 standpoint is that the fine sands and silts are in nearly all cases pink or 

 brown, representing former fluviatile deposition, and the lower beds 

 show more intense pink and reds. This transition in color from the 

 recent to the ancient deposits is to be attributed to the ready partial 

 dehydration which limonite imdergoes with time, warmth, and pressure. 

 The clay bands which recur frequently through tlie section, thougli in 

 part pink, hold many green members. As shown by the present condi- 

 tions, the green clays mark the presence of former lacustrine conditions 

 and point to the significance of clay bands of these colors in red silts or 

 sandstones of other formations. 



Although the climate is arid and the lake occasionally dries up, saline 

 deposits are absent, since the lake wlien at higli level overflows into a 

 lower basin. In general, however, saline deposits would be associated 

 with sedimentation under the climatic conditions which prevail in Persia, 

 and deoxidation of lacustrine deposits would be less effective. 



Ancient example, the Orcadie basin of Scotland. — An example of the 

 application of these color relations to an ancient deposit may be made as 

 follows : In the Lower Old Red Sandstone formations of the Orcadie 

 basin of Scotland the sandstones and non-calcareous shales are domi- 



15 The basin of eastern Persia and Seistan. Carnegie Institution Publication. No. ild, 

 1005. (Also personal communication.) 



