500 PROFESSOR GEIKIE ON THE 
these two varieties, the term anamesite may be retained as a convenient 
synonym for those coarser basalts, where the felspar in well-defined crystals 
plays a leading part. 
1. General External Characters and Mode of Occurrence.—The basalts are 
broadly distinguished from the dolerites by their greater closeness of texture, 
darker colour, tendency to assume a regular columnar form, frequent slagey 
and amygdaloidal character, and their association with tuffs as interbedded 
sheets in the Carboniferous system. The more compact varieties break with a 
splintery conchoidal fracture, are iron-black on the fresh surface, and appear to 
consist of a homogeneous dull substance, in which only here and there a few 
shining crystalline facettes canbe seen. From this extreme graduations can be 
traced to a distinctly porphyritic texture, where the same dark base is crowded 
with crystals, among which prisms of labradorite, and augite and grains of | 
olivine can be seen with the naked eye. The beautiful anamesite of Craiglock- 
hart Hill, near Edinburgh, contains well-formed crystals of augite of the usual 
form, sometimes half an inch long, and crystals of serpentised olivine nearly as 
large. 
The action of the weather, however, has greatly altered the external aspect 
of the basalts. In some cases, especially where columnar, they weather — 
spheroidally, but instead of the thick coating of decayed shells so common 
among the dolerites, they may be found with merely a thin yellow crust, below 
which the rock appears fresh and black. Where they are amorphous, and 
especially where amygdaloidal, they decompose with a very characteristic dirty 
green aspect. Alternations of these two modes of weathering may be observed 
even in the same cliff, as at King Alexander’s Crag, near Burntisland, where 
the bedding of the basalts can be descried even from a distance by means of 
the difference. 
The basalts occur chiefly as interbedded sheets. With the exception of the — 
porphyrites, all the lavas poured out at the surface in the Basin of the Firth 
of Forth during the Carboniferous period were basalts. The hills between 
Bathgate and Linlithgow, those of Burntisland in Fife, and of the neighbour-— 
hood of Edinburgh, offer good examples. 
But the basalts assume also an intrusive form. So far as I have observed, 
they have never been thrust in great sheets among previously formed rocks ; at 
least the rock so thrust has not consolidated in the form of basalt. They oa 
of the Firth of Forth. P 
