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VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
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only a few small intrusive masses. But as the broad anticline of the Firth 
of Tay opens out and allows the lower or pre-volcanic 
members of the Old Red Sandstone to approach the 
surface, another group of bosses emerges from the 
lower sandstones and Hagstones. Some of these 
cover a considerable space at the surface, though 
a portion of their visible area may be due to lateral 
extravasation from adjacent pipes, the true dimensions 
of which are ther eby obscured. Some of the masses 
are undoubtedly sills. In the case of Dundee Law 
we probably see both the pipe and the sill which 
proceeded from it ; the prominent, well-defined hill 
marking the former, while the band of rock which 
stretches from it south-westwards to the shore belongs 
to the latter. The material that forms the bosses and 
sills in this neiglibourhood is generally a dark com- 
pact andesite. The rock of Dundee Law was found 
by Dr. Hatch to show under the microscope “ striped 
lath- shaped felspars abundantly imbedded in a finely 
granular groundmass, speckled with granules of mag- 
netite, lint showing no unaltered ferro- magnesian 
constituents.” Here and there in the same district 
a solitary neck may be observed filled with agglomer- 
ate (Fig. 78). 
The variations in the structure of the Ochil and 
Sidlaw volcanic group will be most easily understood 
from a series of piarallel sections. Beginning on the 
north-eastern or Sidlaw branch of the volcanic band, 
we find the arrangement of the rocks to be as is 
shown in the accompanying figure^ (Fig. 78). As 
is usually the case in this region, the base of the 
volcanic series is here concealed by the fault which 
brings down the Tapper Old Bed Sandstone under 
the alluvial deposits of the Carse of Gowrie. The 
total thickness of the series in this section is about 
2500 feet. The rocks consist of successive sheets 
of andesite of the fiimiliar types, varying in colour 
through shades of blue, purple and red, and in texture 
from a dull compact almost felsitic character to 
more coarsely crystalline varieties. They are often 
amygdaloidal, especially in the upper and lower 
portions of the individual flows. They are not in- 
frequently separated from each other by courses of conglomerate or ashy 
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1 This section and the notes accompanying it have been supplied by Prof. .Tames Geikie, who 
mapped the western half of the Sidlaw range for the Geological Survey. The eastern half was 
mapped by the late Mr. H. M. Skae. 
