386 Baily— Cambrian Rocks and Fossils. 
the authority of the Geological Survey, exhibit between Harlech and 
Dolgelly, according to Mr. Alfred Selwyn, 8,000 feet of thick beds 
of quartz-rock, sandstone, and clay-slates, the lower part of the series 
not being observable; and in the section by Professor Ramsay, across 
the Menai Strait, and over Glyder Fawr, to the north of Snowdon, 
the upper 5,000 feet of the Cambrian series in that part of the country 
is described as consisting of green and purple slates, grits, sandstones, 
and conglomerates ; the Penrhyn and Llanberis slate-quarries being 
worked in a band of slate at the upper part of the series. * 
With respect to the occurrence of organic remains, the author of 
‘Siluria’ observes, ‘No fossils except marine plants or fucoids have 
yet been detected in these rocks of North Wales.’ + 
The Cambrian rocks of the Longmynds are described by the same 
eminent geologist as a range of round-backed hills flanking the 
western side of the road from Ludlow to Shrewsbury, and rising to 
heights varying from 1,400 to 1,600 feet, ranging from NNE. to 
SSW.; they stand boldly out from beneath the surrounding Silurian 
deposits, of which they form the mineral axis, the lowest beds being 
schists or clay-slates, with minute layers of silicious limestone, 
interrupted by bosses of eruptive trap-rocks, and overlain by a vast 
and regular series of hard purple or greenish and grey schistose 
flagstones and silicious grits, often finely laminated, on which the 
rippled surfaces are distinctly visible. t 
The thickness of this great series of strata is estimated by Mr. 
W. T. Aveline, of the Geological Survey, at more than 20,000 feet. § 
Traces of organic life, although obscure, have been discovered in 
some of the fine-grained sandstones and shales of these ancient 
deposits by Mr. J. W. Salter, who describes them|| as consisting of 
the impressions or surface-holes and burrows of Annelides of, he 
thinks, two species, which he names Arenicolites didyma and A. spar- 
sus: these he believes to have burrowed in sand like the Lob-worm 
(Arenicola) of the present day, mentioning their occurrence as being 
in the greatest profusion and distinctly in pairs, indicating the exit 
and entrance holes to the burrows. That paleontologist also describes 
broad undulations, which he considers to be wave-marks, such as 
may now be seen on the sea-shore, and smaller and finer ridges or 
rippled surfaces, which, according to his views, may either represent 
the quiet action of the surf on a level strand, or the agitation of the 
water by wind. Certain rounded markings scattered numerously 
over the surfaces of some of the rippled and other slabs he believes 
to be impressions from rain-drops; and other markings also of fre- 
quent occurrence on the fine sandstones he describes as ‘sun-cracks’ 
or ‘sun-dried surfaces,’ the effect of the sun’s heat upon the ancient 
* J. B. Jukes, Student’s Manual of Geology (1862), pp. 435, 486. 
} Siluria (2nd edit.), p. 27. t Lbid., pp. 22, 23. 
§ See sheets 34 and 36, Horizontal Sections of the Geological Survey of Great 
Britain. 1g 
| Quart. Journ. Geol. Soe. of Lond., vol. xii. p, 248 (1856), and vol. xiii. 
p. 206 (1857). 
