AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 135 
between normal granophyres and quartz dolerites. The iron-ore is all in the form of 
leucoxene and the susceptibility is again low. 
(6.) The Porphyroids represent altered dacites, that is, andesitic rocks of rather 
acid composition. They have been described by Prof. Bonney and Mr. 
and by Prof. Watts.! The specimen tested, from the well-known occurrence at High 
Sharply, was a typical example. It consists of porphyritic quartz, orthoclase, and 
plagioclase set in a crypto-crystalline ground-mass showing flow-structure. The 
phenocrysts often occur as glomero-porphyritic aggregates. The rock is considerably 
cleaved and altered and the dark minerals, which were only present in small amount, 
are replaced by epidote and chlorite. Iron-ores are present as minute specks in the 
ground-mass, and as larger crystals in the glomero-porphyritic aggregates. They are 
now represented by leucoxene. If any magnetite was present it has been completely 
altered, with the result that the susceptibility of this rock was much lower than that 
of any other of the igneous rocks that were tested. 
‘ Q. J. G. S.,’ vol. XXXIV. (1878), p. 199 ; vol. XXXVI. (1880), p. 337 ; and vol. XLVH. (1891), 
p. 78. 
t “ The Geology of Burton 
and Loughborough,” ‘Mem. Geol. Survey,’ 1905, p. 8. 
