BY W. N. BENSON. 609 



lar grains. A small amount of quartz occurs with the felspar, but 

 iron-ores are not developed. 



A very special form of pyroclastie rock is the agglomerate 

 which occurs in Portion 162, Nemingha. As described above, this 

 rock contains a number of rounded pebbles of igneous rock in a 

 tuffaceous matrix. Though several types of rock appear to b-^ 

 represented among the pebbles on macroscopic examination, the 

 miscroseope shows them to be mostly of one type in different stages 

 of alteration. They are porphyrites, with phenocrysts of plagioclase, 

 more or less decomposed, slightly zoned, and, for the most part, 

 determinable as oligoclase. In some of these, the former zoning is 

 strongly marked by the presence of kaolin or dusty matter, though 

 optical tests show the crystal to be of uniform acid composition 

 (cf.29, p723). Augite also forms phenocrysts, but is more or less 

 chloritised, and has a strongly marked outer margin of magnetite. 

 There are smaller phenocrysts of magnetite, and corroded quartz, 

 lying in a very fine-grained felsitic base. In addition, there is a 

 porphyritic spilite, with phenocrysts of albite, in a pilotaxitic to 

 subvariolitic groundmass of albite-laths, with chlorite, ilmenite, 

 magnetite, and titanomorphite. Sometimes there are vesicles 

 filled with calcite. One of these rocks is very similar to the spilite 

 of Tintinhull, or of the Eastern Series. Another rock(135r)) has 

 a rhyolitic appearance, but is quite holocrystalline, and very finely 

 granular. Its composition is that of a magnetite-keratophyre. 

 There are idiomorphic or slightly corroded phenocrysts of acid fel- 

 spar, and a few large accretions of minutely granular magnetite, 

 and glomero-porphyritic quartz-felspar aggregates. The ground- 

 mass is made up of trachytic microlites of felspar, dotted with 

 dusty magnetite, and interspersed with rounded felspar-crystals, 

 which are not really spherulites, as their appearance suggests. 

 These are arranged in bands, and thus give the rhyolitic appear- 

 ance to the rock. The matrix of this rock is composed of crystals 

 and fragments apparently derived from the porphyrites, which 

 form the most abundant inclusions. 



Tlie pyroclastie rocks in the Upper Middle Devonian Series are 

 similar to those in the Lower Middle Devonian, but are generally 



