ARENIG OR STIPER-STONES GROUP. 
475 
and Trinudeus (Figs. 552 , 553 ) form a marked feature of 
the rich and varied Trilobitic fauna of this age. 
Beneath the black slates above described of the Llandeilo 
formation, graptolites are still found in great variety and 
abundance, and the characteristic genera of shells and trilo- 
bites of the Lower Silurian rocks are still traceable down¬ 
ward, in Shropshire, Cumberland, and I^orth and South 
Wales, through a vast depth of shaly beds, in some districts 
interstratified with trappean formations of contemporaneous 
origin ; these consist of tuffs and lavas, the tuffs being formed 
of such materials as are ejected from craters and deposited 
immediately on the bed of the ocean, or washed into it from 
the land. According to Professor Pamsay, their thickness 
is about 3300 feet in North Wales, including those of the 
Lower Llandeilo. The lavas are feldspathic, and of porphy- 
ritic structure, and, according to the same authority, of an 
aggregate thiokness of 2500 feet. 
Arenig or Stiper-Stones Group {Loiver Llandeilo of Mur- 
chiso7i ).—Next in the descending order are the shales and 
sandstones in which the quartzose rocks called Stiper-Stones 
in Shropshire occur. Originally these Stiper-Stones were 
only known, as arenaceous quartzose strata in which no or¬ 
ganic remains were conspicuous, 
except the tubular burrows of an¬ 
nelids (see Fig. 563 , Ai^enicolites 
Imearis)^ which are remarkably 
common in the Lowest Silurian in 
Shropshire, and in the State of New 
York, in America. They have al¬ 
ready been alluded to as occurring 
by thousands in the Silurian strata 
unconformably overlying the Cam¬ 
brian, in the mountain of Queenaig, , . . tT A 
in Sutherlandshire (Fig. 82 , p. 112 ). beds, stiper-stones. 
I have seen similar burrows now Parting between the beds, or 
made on the retiring of the tides Fanes of bedding. 
in the sands of the Bristol Channel, near Minehead, by lob¬ 
worms which are dug out by fishermen and used as bait. 
When the term Silurian was given by Sir R. Murchison, in 
1835 , to the whole series, he considered the Stiper-Stones as 
the base of the Silurian system, but no fossil fauna had then 
been obtained, such as could alone enable the geologist to 
draw a line between this member of the series and the Llan¬ 
deilo flags above, or a vast thickness of rock below, which 
was seen to form the Longmynd hills, and was called “ unfos- 
siliferous gray wacke.” Professor Sedgwick had described, in 
