BY W. N. BENSON. 



013 



Middle Devoyiian Cherts and Claystones. 



The sedimentary rocks of the Middle Devonian Series have 

 few features of petrological interest. The general characters 

 were described by Messrs. David, Pittman, and CardO), and the 

 writer(16). Here and there, they are enriched with tuffaceous 

 matter, where zones of larger grainsize composed of fragments of 

 quartz, felspar, or cryptocrystalline felsite, are interstratified 

 with the normal claystone, into which they gradually shade away. 

 In none of the rocks which the writer has studied microscopi- 

 cally, has he been able to recognise the presence of minerals 

 which ordinarily characterise contact-metamorphism, nor does it 

 appear probable that they would be developed, if the intrusions 

 of the pyroclastic material took place in the manner here de- 

 scribed, which would involve an intrusion at a fairly low tem- 

 perature. 



Considerably altered rocks occur about the granite; the most 

 intensely altered are those actually included in the granite in 

 Portion 66, Tam worth. These have abundantly developed biotite, 

 and are interleaved with narrow bands of pegmatite, and granite. 

 A less altered type (1133) is a fine-grained quartz-schist, with 

 minute, chloritised flakes of biotite, and fragments of orthoclase 

 and andesine. 



The chemical composition of the sedimentary rocks will be 

 seen in the table herewith, of analyses by Mr. MingayeO). 



Assay number ('97). 



SiO„ ... 

 Al,03 . 

 Fe,03 

 xVlgU ... 

 CaO ... 

 NaoO ... 

 K,0 .. 

 H^O .. 

 H2O- 

 CO2 ... 

 P,0« ... 

 SO3 ... 

 Fe82 .. 

 MuO .. 

 Organic matter 



1233 



18 05 

 349 

 4-87 

 1-65 



38-70 

 29 

 0-44 

 1-42 

 0-80 



3015 

 0-34 

 tr. 

 0-24 

 tr. 

 tr. 



100-44 



