VOLCANOES OF THE LOWER OLJ) RED SANDSTONE book v 
^o8 
parent focus of discharge. It is impossible to tell 
how much of the volcanic series is here concealed by 
the unconformable overlap of the yonnger formations. 
A section across the centre of the Ochil chain/ 
from I.)iinning in Stratliearn to the Crook of Devon 
and the Fife Coal-field, gives the structure which is 
generalized in Fig. 80. rVt the north end the vol- 
canic series is found to be gradually split up into 
separate lava-sheets until it dips under the red sand- 
stones of Strathearn. Traced southwards the rocks 
become entirely volcanic. Some of their most con- 
spicuous and interesting members are pale felsitic 
tuffs, which occupy a considerable, tract of ground 
about Craig Ilossie, south-east of Auchterarder. As 
the dip gradually lessens the harder lavas are able to 
spread over wider tracts of ground, capping the hills 
and ridges, while underneath them thick masses of 
tuff and conglomerate are laid bare in the valleys. 
A number of bosses of orthophyre rise through these 
rocks and are accompanied by many veins and dykes 
of similar material. It is not improbable that some 
of these bosses, as already suggested, may represent 
r-ents. They are especially prominent among the hills 
due south of Auchterarder. One of these eminences, 
known as the Black Mailer, is composed of a tj’pical 
orthoclase-felsite without mica. Another, about four 
and a half miles further south, forms the conspicuous 
summit of Ben Shee overlooking Glen Devon, and 
consists of a similar rock with a characteristic platy 
strncture. 
Xo necks of agglomerate have been observed in 
this part of the chain. It will be seen from the 
section that the lowest visible parts of the Ochil 
volcanic series are here truncated by a fault which 
brings in the lower part of the Carboniferous system. 
By a curious conjuncture, immediately on the south 
side of this fault, a band of tuff appears, lying on 
the platform of the Carboniferous “ plateau -lavas," 
to be hereafter considered, and passing below the 
well-know'ii Ilurlet seam of the Carboniferous Lime- 
stone, while through these strata rises one of the puys 
belonging to the second phase of volcanic activity in 
Carboniferous time in Scotland. 
The best sections to show the nature and sequence 
* The central portion of the Ochik was mapped for the Geological Survey by Mr. B. IS'. Peach. 
Prof. James Geikie, Prof. J. Yomig, Mr. R. L. Jack ami myself. 
