36 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
foliateii appearand by the development of pale leek-green folia, which show 
0 “ 2 Pamll f el With the beddiu 8- The rock then presents one 
o the usual appearances ot schalstein. This structure seems obviously due 
to mechanical movement along the planes of stratification 7 
-bands of black chert and cherty shale are interpolated amom- the tuffs 
^hySL COnt8in ' h t ere r d there n0dukr lumps ° f simila1 ' black impure' 
tTe cLbonifr m f § aSSOCmtiou like that alluded to as occurring in 
of the raXr “T °* ^ Ik1 ° ° f Man > and Iike ^ occurrence 
described " ^ ^ the W Sil ™ volcanic series already 
twomiles^Tfl 0 Vi? ^ ° f the Teign cai1 be followed for about 
v ch are dSincf 1 , 7 -terstratified among the dark Culm-measures, 
uJncfi aie distinctly seen dipping under and overlying it. 
general Ah Mahon has recently shown what may be done by careful 
mid detailed examination of the ground broadly sketched in by De fa Beche 
Man tf 0I ’ M Udy r StPlp ° f “ greenstoiie ” shown on the Geological Survey 
52 “ a 'out three and a half miles along the north-west margin of 
e Gmtmooi gramte. He has found that wliat is represented under one wash 
of colour on that map includes both tufts and lavas. The tufts in spite of 
the alteration which they appear to have undergone from the proximity of 
he great granite mass, are found by microscopic investigation to Tide 
ip oi fine volcanic dust containing minute lapilli of various lavas Some 
mes as many as six or seven different kinds of lava may be represented in 
e same microscopic slide. These include felsitie or rhyolitic and trachytie 
Wftf the tuff W1 1 fr T ei f ° f ^ gks8p laVa ful1 of magnetite dust. 
With the tuffs are intercalated sheets of felsite and trachyte. In the 
/ m® dlstue * coarse v °lcamc agglomerate occur, made up of blocks of different 
lavas and pieces of different sedimentary rocks. 2 
These observations are of special interest, inasmuch as they point to the 
eruption of a much more acid series of volcanic lavas and tuffs than h fd 
previously been known to exist in the Culm-measures. Until the mound 
ms been more accurately mapped, it is impossible to say whether these 
locks are older or younger than those that lie around Brent Tor a few 
mles to the south-west. General M'Mahon has noted the pre^nce "f 
more basic eruptive rocks in the same district. He specially cites ti e 
idTalk “."“f °f i»to . ipentinou, mi 
To, folLno H k T "7 ‘ ‘ y ” mrk * he aclual old Brent 
■ZZ H . observations on the influence of the Dartmoor granite in 
ug new mineral rearrangements in the igneous rocks of the Culm 
measure series are full of interest. 
‘ p “”: 1 h * v * *««• * 
««.«■ jou,,. TCSTm ” .iL* id 
Quart. Journ. Geol. Soc. vol. 1. (1894), p. 338 . 
