12 The Irish Naturalist. 



the quartz-rocks are intrusive and d^'ke-like, not ordinary 

 altered sediments. But when we consider how mau}^ thrusts 

 and squeezes these ancient rocks have had to bear, it seems far 

 more probable, as Prof. Sollas has shown/ that the hard 

 resisting quartzites have been broken up under the stress, and 

 that the softer and more yielding cla3\s have flowed round 

 about them ; so that a mass of quartzite may be found quite 

 out of its proper position, and even cutting across the .strati- 

 fication of the shales, instead of conforming to it. The real 

 sandy origin of these quartzites can again and again be 

 proved, as may be seen in the gritty mass on the summit of 

 the great Sugarloaf, or as Mr. T. Oldhanr pointed out as early 

 as 1844, when he produced pebbles from the quartz-rock of 

 Bray. The dislocations and faults'* are proofs of the power of 

 the earth-movements, and the microscope, as Prof. Sollas has 

 shown, reveals the same thing in the minutest details, the old 

 sand-grains being ground to powder on their edges and 

 becoming cracked and elongated, as in all hard rocks in which 

 the particles have been forced into a kind of flow. 



If, then, these basement-rocks of Co. Dublin are merely 

 altered marine deposits, there must have been a land formed 

 of still earlier rocks, on the shore of which the}' were laid 

 down. This .shore still remains concealed, but Prof. Sollas 

 has already been engaged in elucidating its position. Very 

 possibly it was in part made of the old crushed and foliated 

 schistose rocks, which are exposed in the western counties, 

 and which underlie the Cambrian in their typical region, 

 Wales. 



The Cambrians of Cambria contain fossils, limited in species, 

 but covering a fair range of forms. We have abundant 

 Brachiopods, particularly Lingulclla, the close ally of the living 

 Lin 021 la ; rare lyamellibranchs and Gastropods ; Thcca, which 

 may prove to be something between a Gastropod and a Cepli- 

 alopod ; Orthoceras, the old straight representative of the 

 Pearly Nautilus ; and numerous Trilobites, the well-known 

 animals which died out in the Carboniferous period, but 

 which seem to have been the lords of creation in early Cam- 

 brian times. Since they probably stand as links between the 

 Crustacea and the Arachnida, and would doubtless be disowned 

 nowadays both by the scorpion and the woodlouse, it is clear 

 that the life of the Cambrian period ma}^ be regarded as primi- 

 tive, however abundant. Primitive, but in no wa}' primordial; 

 for alread}^ geologists have fixed the base of the Cambrian 

 rocks b}^ the presence of a particular Trilobite, Olcncllus, and 

 the "Olenellus-fauna," or group of species found in association 

 with OlcncUus, marks an horizon below which research is being 



"^ Proc. Roy. Dublin Soc. vii., part 3. 



2 " On the rocks at Bray Head," G. S. D. iii., p. 60. 



^See Map of Bray Head, by G. A. Kinahan. G. S. I. vi., pi. vii. 



