Parr II. Sect. iv. § 2.] CARBONIFEROUS. 739 
massive limestones dwindle down and are replaced by thick courses of 
yellow and white sandstone, dark shale, and seams of coal and ironstone, 
among which only a few thin sheets of limestone are to be met with. 
Scottish geologists have divided the lower half of their Carboniferous 
system into two well-marked series—the Calciferous Sandstones and the 
Carboniferous Limestone. The Calciferous Sandstone series is com- 
posed of two groups of strata—the lower of which or Red Sandstone 
group consists of red, white, and yellow sandstones, blue, grey, green, 
and red marls or clays, while the upper or Cement-stone group is made 
up of white and yellow sandstones, blue and black shales, thin coals, 
seams of limestone and cement-stone, and abundant volcanic rocks. The 
red sandstones pass down into the Upper Old Red Sandstone, with which 
indeed they might be classed, and from which they differ merely in the 
less intensity of their colour, in the frequent grey and purplish tints 
they assume, and in the absence of the deep brick-red marls so marked in 
the Upper Old Red Sandstone. In the west of Scotland, as above (p. 716) 
stated, there occur among the red sandstones (some of which contain 
Upper Old Red Sandstone fishes) bands of limestone full of true Car- 
boniferous Limestone corals and brachiopods. Hence it is evident that 
the, Carboniferous Limestone fauna had already appeared outside the 
British area before the final cessation of the peculiar conditions of 
sedimentation of the Old Red Sandstone period. It was not how- 
ever until these conditions had disappeared that the sea began to invade 
the lakes and creep over the sinking land of this part of Britain, and to 
bring with it the abundant Carboniferous fauna. The Calciferous 
Sandstones of Scotland represent a phase of sedimentation contempo- 
raneous with the deposition of the Lower Limestone Shale and lower 
portion of the Carboniferous Limestone of England. 
One of the most singular features of the Lower Carboniferous rocks of 
Scotland is the prodigious abundance of the intercalated volcanic rocks. 
So varied indeed are the characters of these masses and so manifold and 
interesting is the light they throw upon volcanic action that the region 
may be studied as a typical one for this class of phenomena. (See Book IV. 
Part vii. Sect. i.) Sections are abundant inland on the sides of the hills 
and in the stream-courses, while along the sea-shore the rocks have been 
admirably exposed. The most persistent zone of volcanic rocks in the 
whole of the Scottish Carboniferous series is that which succeeds the lower 
or red sandstone group of the Calciferous Sandstones. Composed of 
successive sheets of porphyrites and tuffs, it sweeps in long isolated 
ranges of hills from Arran and Bute on the west to the mouth of the 
estuary of the Forth on the east, and from the Campsie Fells on the north 
to the heights of Ayrshire and still further south in Berwickshire, 
Liddesdale, and the English border. These volcanic sheets sometimes 
reach a thickness of 1500 feet. That they belong to the Carboniferous 
system is shown by the occurrence of shales and sandstones (with Car- 
boniferous plants) at their base. They show that the early part of the 
Carboniferous period in Scotland was marked by a prodigious volcanic 
activity, followed by the prolonged subsidence required for the accumula- 
_tion of the Carboniferous system. 
Above this volcanic zone lies the Cement-stone group or upper sub- 
division of the Calciferous sandstones. In Berwickshire and the west of 
Scotland it consists of thin-bedded white, yellow, and green sandstones, 
grey, green, blue, and red clays and shales, with thin bands of pale 
3B 2 
