PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 27 



and 35379), and Lake Labontoii, Nevada (35378). Concerning the occur- 

 rence of these last Mr. Russell writes:* 



"Among The Needles the rocky capes are connected by crescent-shaped beaches of 

 clean, creamy sands, over which the summer surf breaks with soft murmurs. These 

 sands are oolitic in structure, and are formed of concentric layers of carbonate of lime 

 which is being deposited near where the warm springs rise in the shallow margin 

 of the lake. In places these grains have increased by continual accretion until they 

 are a quarter of an inch or more in diameter, and form gravel, or pisolite, as it would 

 be termed by mineralogists. In a few localities this material has been cemented into a 

 solid rock, and forms an oolitic limestone sufficiently compact to receive a polish. 

 No more attractive place can be found for the bather than these secluded coves, with 

 their beaches of pearl-like pebbles, or the rocky capes, washed by pellucid waters, 

 that offer tempting leaps to the bold diver." 



Such forms as these may or may not show a nucleus. It seems safe 

 to assume that such a nucleus at first in all cases existed, though it may 

 be in microscopic dimensions only. A shell nucleus is shown in the clay 

 ironstone concretion from Kansas (73454), and fragmental nuclei of 

 siliceous sinter in the concretionary nodules from the geysers of the 

 Yellowstone National Park (12888). 



Under (b) are shown concretions composed of mineral particles in a 

 finely fragmental condition, and which have as in the last case segre- 

 gated contemporaneously with the formation of the material iu which 

 they occur. Here are included a series of clay concretions from the head- 

 waters of the Connecticut Eiver (38425) ; from the Yellowstone Lake 

 (12895) ; and from various beds of brick and potters' clay iu New Eng- 

 land. In certain of these the presence of a nucleus is plainly evident, 

 those from Orono, Maine (369G5), having formed about stems of grasses; 

 those of Jefferson County, Tennessee (38357), about small shells.t 



The secondary concretionary forms (B) are likewise susceptible of sub- 

 division on precisely similar grounds. Under the head of chemical de- 

 posits would come such forms as flint nodules in chalk (3G012) and the 

 agates formed in cavities in trap rock (69569). Such do not in all cases 

 show a concentric structure and might perhaps be better termed secre- 

 tions than concretions, and classed with mineral veins. Under the 



* Geological History of Lake Lahonton, a Quaternary Lake of Northwestern Nevada, 

 Monograph XI, U. S. Geological Survey. 



* The manner in which concretions of this nature are formed was shown in a very 

 interesting manner a few years ago during the process of the work of filling in the 

 so-called Potomac flats, on the river front at Washington, District of Columbia. For 

 tin; double purpose of raising the flats and deepening the channel gigantic pumps 

 were employed which raised the sediment from the river bottom in the form of a very 

 thin mud and forced it through iron pipes to the flats, where it flowed out spreading 

 quietly over the surface. The material of this mud was mainly fine siliceous sand 

 and clay intermingled with occasional fresh water shells and plant debris. As this 

 mud flowed quietly from the mouth of the pipe and spread out over the surface the 

 clayey particles began immediately to separate from the siliceous sand in the form of 

 concretionary balls, and in the course of a few minutes these would grow to be several 

 inches in diameter. Such, owing to tin; rapidity of their formation, contained a largo 

 amount of sand and shells, though clayey matter predominated, 



