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THE CAMBRIAN VOLCANOES 
BOOK III 
visible part of the Caldecote volcanic series to the base of the Coal-measures 
is probably between 2000 and 3000 feet. 
There can be iio doubt as to tlie geological position of the dark fossil- 
iferous shales and their underlying cpartzite. The fact that the basement 
conglomerate of the quartzite is partly made up of the underlying volcanic 
series may possibly mark a wide difference of age between them, and may 
indicate that the eruption of the tuffs took place long before Upper 
Cambrian time. On the other hand, the tuffs have the same strike and 
angle of dip with the quartzite, and as Professor Lapworth admits, the 
break between them may not be of great moment. It is at least certain 
that the intrusive sills of the district are later than the tuffs^ and later also 
than the sedimentary Cambrian groups. 
