BY W. N. BENSON. 583 



tonic structures and the thickness of the formations involved might 

 be exactly determined, has been in a large degree unsuccessful. It 

 has, however, been proved that some faulting is present, producing 

 a greater degree of repetition of strata than was assumed in the 

 previous estimate of the total thickness of the series, though the 

 thickness of the individual beds of limestone, claystone, and tuff, 

 and the total apparent thickness, were measured with great care. 

 As yet, however, it is quite impossible to determine the exact 

 amount which must be deducted from that total (9,260 feet). 

 Plotting the old line of section on the new map, we find that the 

 lower portion of the Baldwin Agglomerates on Spring Creek was 

 included in the earlier total. The agglomerates here are as finely 

 granular as much of the pyroclastic matter in the Middle Devonian 

 Series, and they would naturally be classed with the latter, before 

 detailed mapping had shown their connection with the deposit of 

 large boulders on Cleary's Hill. To obtain the total apparent 

 thickness of the Devonian rocks in this district, we must, therefore, 

 add the remainder of the Baldwin Agglomerates, about 1000 feet 

 and the thickness of the Barraba mudstone, apparently about 2400 

 feet. The columnar section given in Fig. 1 shows the relative thick- 

 ness of the various subdivisions of the Devonian Series as far as 

 can be ascertained at present. 



Conditions attending the formation of the Badiolarian Bocks. 



As this district is frequently cited as a classical example of the 

 development of a series of radiolarian rocks in comparatively shal- 

 low water, the subject should not be passed over in the present 

 communication ; but, as further studies are in progress in adjacent 

 areas, where similar rocks are developed, and much remains to be 

 investigated, we will merely note the bearing of the new observa- 

 tions on the views of the authors who studied the question here pre- 

 viously. According to them, the radiolarian rocks "were deposited 

 in clear seawater, which, though sufficiently far from land to be 

 beyond the reach of any but the finest sediment, was, nevertheless, 

 probably, not of very considerable depth"(9). This conclusion 

 was based on the following considerations: — (1) The presence of 

 ripple-marking, (2) the abundance of plant-remains indicating the 



