806 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



growth of coral colonies, and the same was probably true of the cryptozoa. 

 The following illustrations have been called to the writer's attention 

 by Professor Schuchert, although three of the photographs are by P. E. 

 Raymond, E. S. Bassler, and M. Y. Williams, respectively, who have 

 kindly given permission for their publication. 



The first, plate 44, figure 2, from the Middle Ordovician, shows the upper 

 ] cgular beds of the Lowville and the nodular lower beds of the Black River 

 limestone. The Lowville," in the older nomenclature, was known as the 

 bird's-eye limestone, from the preservation in it of the casts of vertical 

 stems of marine plants, which seen in cross-section give the bird's-eye 

 effect. The individual laminae were therefore deposited rapidly and in 

 shallow water. The formation consists of blue or dove-colored fine- 

 grained limestones Avitli beds of thin shaly limestone and shows an extra- 

 ordinary persistence of lithologic characters over an area of 500,000 square 

 miles, mainly east of the Mississippi River. A marked disconformity is 

 pointed out by Ulrich as existing at the base of the Lowville, as it rests 

 in places on the very similar, but far lower, Stones River limestone of 

 the Lower Ordovician. Some of these limestones, probably including the 

 Lowville, exhibit mud-cracks in many beds and in widely separated locali- 

 ties. The writer has also, but more rarely, found rain-printed surfaces. 

 Ilie Lowville is, however, very fossiliferous in certain layers; so that, 

 although the waters were shallow and repeatedly withdrawn, they were 

 typical marine watei-s. The oscillations must have been very small in 

 amount and were perhaps mostly due to climatic factors, the seas forming 

 marine playas. The overlying, or Black River, limestones are nodular 

 and in places cherty, indicating a different character of deposition and 

 probably much solution of lime in order to concentrate the silica. No 

 large time elapsed, however, between the two formations, as Schuchert 

 states in a ;f)ersonal communication that in the locality shown by the 

 photograph he could not make out a faunal break at the top of the Low- 

 ville, but that there was clearly a depositional break, not a disconformity, 

 however, in the generally accepted sense. 



Referring to the photograph, the beds which are marked A, B, C con- 

 stitute the upper Lowville ; D, the Black River. There is exhibited here 

 a striking series of rhythms. A major rhythm is represented by B, con- 

 sisting of three subordinate beds. Each subordinate bed in B is subdivided 

 in turn into three and these in their turn into thin laminae. The weather- 

 ing brings out a cliange by a series of steps from A to B to C, the latter 

 consisting of six beds and being apparently equivalent to two rhythms of 

 the value of A and B. The subordinate rhythms of B are almost absent 

 from C. It is not thought that stress should be placed on the precise 



