CHAP. XI.X 
THE OCHIL AND SIDLA W ERUPTIONS 
307 
sandstone and grit. Of these intercalations four are ot sufficient thickness 
and persistence to he mapped, and are shown on the Geological Survey 
Sheet 48. The stones in the conglomerates vary up to blocks two feet in 
diameter, and consist chiefly of andesites, hut include also some pink felsites 
and pieces of greenish hardened sandstone. Generally they are more or 
less well-rounded ; but occasionally they become angular like those of vol- 
canic agglomerates. 
One of the most interesting features in this section is the neck which at 
Ov'er llurdie rises through the volcanic series. Oval in form, it measures 
630 yards in one diameter and 350 in another, and is filled with pinkish 
granular tuff, full of andesitic lapilli and blocks. A much smaller neck of 
similar material lies about 100 yards further to the south-west. There 
seems no reason to doubt that these necks mark two of the volcanic vents 
belonging to a late part of the volcanic history of the district. 
The structure of the Sidlaw range is repeated among the hills of east 
Fife on the southern side of the great anticlinal fold.^ Thus a section from 
near Newburgh on the Firth of Tay southward to near Auchtermuchty in 
Stratheden gives the arrangement of rocks shown in Fig. 79. In this 
Fio. 79.— Section across the Eastern Ochil Hills from near Newlinrgh to near Aueliterniuchty. 
1 Lower Old Red Sandstones and conglomerates ; 2. Andesite lavas ; S. Volcanic conglomerates ; 4. Upper Old 
* Red bandstoiift. 
traverse a thick mass of fragmental material occurs in the higher part of the 
series of volcanic rocks. Though on the whole stratified and forming a 
group of conglomerate-beds between the lavas, the material is in places an 
amorphous agglomerate of volcanic blocks varying in size np to two feet in 
diameter. These portions show abundant angular and snhangular blocks, 
many of which, after having undergone some attrition, have been finally 
broken across before reaching their present resting-places.^ Sharply fractured 
surfaces can he picked out of the felspathic ashy matrix. The stones are 
chiefly varieties of andesite, but they include also pink felsites and pieces of 
some older fine-grained tuff. 
These frac^meiital materials form a local deposit about nine miles long, 
and probably not less than 1700 feet thick. They are partly interstratified 
with flows of andesite. Though, from the rounded forms of some of the 
pebbles, wave -action may be interred to have been concerned in their 
accnnrirlatiorr, they seem to be nrainly due to volcanic explosions. No trace, 
however, has beetr foirnd of the veirt frorrr which the eruptiorrs took place. 
Not improbably its site lies sonrewhere to the south irr the area now concealed 
under the Upper Old Eed Sandstone arrd Carboniferous formations. The 
large size of marry of the blocks suggests that they do rrot lie far from their 
1 The eastern part of the Ochils was mapped for the Geological Survey by Mr. H. H. Howell 
and Mr. B. N. Peach. 
