BY W. N. BENSON. 607 



sence of stratification and of organic remains are the only means 

 of distinguishing', in liand-specimen or microscope-slide, an inter- 

 stratified from an intrusive tuff. 



All along the margin of the granite, these rocks are found to be 

 more or less altered. This metamorphism has been described in 

 general terms in a previous paper (16), and very little additional 

 information has been obtained from the present more detailed 

 study. Generally, the effect has been to convert all the ferro- 

 magnesian minerals present into actinolite, which may be scattered 

 all over the rock in secondary sheaf -like recrystallisations. The 

 ilmenite becomes titanomorphite, and the iDlagioclase, clear albite, 

 which may be more or less erystalloblastic. In more altered rocks, 

 the felspar returns to andesine, which has been found in several 

 instances, particularly in Portion 118, Nemingha. This accords 

 with the behaviour of the spilitic rocks about the granites of 

 Devonshire(27). Erdmannsdorfer concludes, from his study of 

 the diabases about the granite of the Harz Mountains, that, where- 

 as the presence of basic felspar is an index that the rock has suf- 

 fered contact-metamorphism, types with acid felspar result from 

 dynamic metamorphism(28, p. 73); this conclusion cannot, how- 

 ever, be applied satisfactorily to the rocks of the Tamworth 

 district. 



The most altered forms of spilitic rocks are those in Sections 

 113 and 123, Nemingha. Macroscopically, they are very like the 

 garnet-bearing contact-altered spilites of Walkhampton in Devon- 

 shire, which were described by Messrs. Dewey and Flettl27). 

 They are banded dark green rocks, with long lenticular patches 

 of quartz and felspar mosaic, probably representing former fel- 

 spar-phenocrysts, and long bands of epidote and brown garnet, 

 together with a little secondary acid felspar. The dark base con- 

 sists of very fuiely matted chlorite and actinolite, while there are 

 also streaks of secondary magnetite. It is not clear whether these 

 are altered vesicular, porphyritic spilites, or pyroclastic rocks. The 

 former is the more probable. Adjacent to these, is the amphibolite 

 (altered dolerite) described above. 



Other types of rock occur, of which a few examples will suffice. 

 A rock (1134), which occurs in Portion 173, Tamworth, is an 



