CHAP. XXXI 
A YRSH1RE AND NITHSDALE 
57 
year 1866. 1 Since that time the progress of the Survey has extended the 
detailed mapping into Nithsdale and Annandale, but without adding any new 
facts of importance to the evidence furnished by the Ayrshire tract. 2 
The materials erupted by the Scottish Permian volcanoes display a very 
limited petrographical range, contrasting strongly in this respect with the 
ejections of all the previous geological periods. They consist of lavas generally 
more or less basic, and often much decayed at the surface ; and of agglomer- 
ates and tuffs derived from the explosion of the same lavas. 
The lavas are dull reddish or purplish-grey to brown or almost black rocks ; 
sometimes compact and porpliyritic, but more usually strongly amygdaloidal, 
the vesicles have been filled up with calcite, zeolites or other infiltration. 
The porphyritic minerals are in large measure dull red earthy pseudomorphs 
of haematite, in many cases after olivine. These rocks have not yet been 
fully studied in regard to their composition and microscopic structure. A few 
slides, prepared from specimens collected in Ayrshire and Nithsdale, examined 
by Dr. Hatch, were found to present remarkably basic characters. One from 
Mauchline Hill is a picrite, composed chiefly of olivine and augite, with a 
little striped felspar. Others from the Thornhill basin in Dumfriesshire show 
an absence of olivine, and sometimes even of augite. The rock of Morton 
Castle consists of large crystals of augite and numerous grains of magnetite 
in a felspathic groundmass full of magnetite. Around Thornhill are 
magnetite-felspar rocks, composed sometimes of granular magnetite with 
interstitial felspar. Throughout all the rocks there has been a prevalent 
oxidation of the magnetite, with a consequent reddening of the masses. 
The pyroclastic materials consist of unstratified agglomerates and tubs 
generally found in necks, and of stratified tuffs, which more or less mingled 
with non -volcanic material, especially red sandstone, are intercalated among 
the bedded lavas or overlie them, and pass upward into the ordinary 
Permian red sandstones. 
The agglomerates, though sometimes coarse, never contain such large 
blocks as are to be seen among the older Palaeozoic volcanic groups. Iheii 
composition bears reference to that of the bedded lavas associated with them, 
pieces of the various basalts, andesites, etc., which constitute these . lavas 
being recognizable, together with others, especially a green, finely-vesicular, 
palagonitic substance, which has not been detected among the sheets of lava. 
In general the agglomerates contain more matrix than blocks, and pass 
readily into gravelly tuffs. A series of specimens collected by me from necks 
which pierce the Dalmellington coal-field has been sliced and examined 
under the microscope by Mr. Watts, who finds it to consist of basic 
tuffs, in which the lapilli include various types of olivine-basalt, sometimes 
glassy, sometimes palagonitic, and occasionally holocrystalhne, also pieces o 
grit, shale and limestone. In one case a crinoid joint detached from its 
1 Heal. Mag. for 1866, [i. 243 ; and Murchison’s Siluria, 4th edit. (1867), p. 332 
^ The rocks are shown in Sheets 9, 14 and 15 of the Geological Survey of Scotland to , 
and their accompanying Explanations, reference is made. The Ayrshire basin was mapp y ' 
the necks in the Dalmellington ground by Mr. James Geikie, the Nithsdale area by Mr. R. L. Jack, 
Mr. H. Skae and myself. 
