Study of a geological map of the world will shew that extensive regions, such as parts of Scandinavia, many tracts of Central Europe, a large area in Canada, and a considerable portion of Brazil and the adjoining countries are occupied by crystalline schists, which underlie the oldest known sedimentary strata in those places. These crystalline schists form the floor upon which the sediments constituting the bulk of the geological column rest, and it is necessary that we should know something of the character of this floor. Other rocks which can be definitely proved to be of Precambrian age are often found associated with the crystalline schists, and these associated rocks have often undergone more or less alteration subsequently to their formation. The difference between the coarser types of crystalline schists and these associated rocks is sometimes so marked that geologists have necessarily paid attention to it, and separated the two groups of rocks; the term Archæan has been used by some geologists to include the crystalline schists, and Eparchæan for the associated rocks of known Precambrian age, but though this separation may sometimes be effected, there are cases when it is impossible to draw any sharp line of demarcation between 'Archæan' and 'Eparchæan' types.
In the present state of our knowledge, a chronological classification of the Precambrian rocks when applied to wide and distant regions is destined to break down, and it will be convenient if we consider at some length the features of the Precambrian rocks of a particular region, and apply the knowledge thus gained to a study of Precambrian rocks of other areas, and to a consideration of our knowledge of the Precambrian rocks as a whole. In doing so, the term 'crystalline schists' will be used somewhat vaguely with reference to a complex of schistose rocks of which the mode of origin cannot be fully determined. We may take our own country as a region where a good development of the Precambrian rocks occurs.
A few explanatory remarks concerning the mode of detection of Precambrian rocks may not be amiss. If any true organisms have been hitherto discovered amongst the rocks formed before Cambrian times they are valueless as a means of correlating rocks, and accordingly lithological characters only are available in attempting to correlate the rocks of one area with those of another. Those who have read the preceding chapters will have gathered that comparisons founded on similarity of lithological character are not so valuable as those made after careful scrutiny of the fossils of strata, but they are by no means valueless, and when the rocks of two areas which are not far distant from one another present close lithological resemblances, their general contemporaneity may be inferred with some degree of certainty.
It is only when we get the lowest Cambrian strata overlying earlier rocks that we have absolute proof of the Precambrian age of the latter, and it is necessary, therefore, that we should have some definite lower limit to the rocks of the Cambrian system. It is now generally agreed that that limit shall be drawn at the base of a group of rocks containing what is known as the Olenellus-fauna, which will be considered at greater length in the next chapter, and it will be well, if the term Cambrian be not in future applied to any rocks beneath the ones containing the relics of this fauna, for otherwise there is danger of the indefinite downward extension of the Cambrian system. We need not be surprised to find great thicknesses of rock below the rocks containing the Olenellus-fauna, and passing upwards with complete conformity into those rocks; nevertheless, if it can be shewn that the Olenellus-fauna had not appeared during the deposition of the underlying group, the rocks of that group should be termed Precambrian. A case of this nature has not yet been detected in our area, and all the rocks which have been proved to be Precambrian in Britain are separated from the overlying Cambrian rocks by a physical break, though that break is not necessarily very large, and in some districts is probably of little importance. Hitherto the Olenellus-fauna has been detected in Ross, Warwickshire, Shropshire, Worcestershire and probably in Pembrokeshire, and the rocks underlying the Olenellus-beds in those counties can be proved to be Precambrian (i.e. if the Olenellus-age of the Pembrokeshire rocks be ultimately established, and the researches of Dr Hicks tend to prove that it will almost certainly be done). It will be convenient if we take the instances where the age of the rocks can be proved with certainty or with a considerable degree of probability first, and then consider the examples of rocks which are found below Cambrian strata, though these have not hitherto yielded the Olenellus-fauna, concluding with a notice of rocks which have been claimed to be of Precambrian age on account of their lithological characters, though they are not now seen to be immediately succeeded by strata appertaining to the Cambrian system.
Commencing with the region where we have the greatest development of the known Precambrian rocks, namely Ross, Sutherland and the Hebrides, we may explain the general relationship of the rocks by means of a generalised section ([Fig. 15]).
Fig. 15.
The lowest rocks a are crystalline schists, they are succeeded by a set of arenaceous rocks b known as the Torridonian beds, which rest unconformably upon the upturned edges of the crystalline schists, whilst the Cambrian rocks, c, rest with another unconformity sometimes upon the partly denuded Torridonian beds, or where the latter have been completely removed, as on the right side of the figure, directly upon the crystalline schists, thus presenting an example of unconformable overlap. The occurrence of the Olenellus-fauna in the basement beds of the Cambrian system near Loch Maree, proves the Precambrian age of the Torridonian strata, whilst the unconformable junction between the latter and the crystalline schists indicates that we are here dealing with two distinct sets of Precambrian rocks, one of Eparchæan and the other of Archæan type.