Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note iv. § 8.

Primary strata not primitive.

152. An account of the facts referred to § 8, may be found in Hutton's Theory, vol. i. p. 332, &c. To what is there said, of the shells contained in the primary limestone of Cumberland, I must add, that I have since had an opportunity of verifying the conjecture, that the limestone rock, in which the shells were found, near the head of Coniston Lake, is part of the same body of strata, where shells were found, in a quarry between Ambleside and Low-wood. The limestone of that quarry contains several marine objects; it is in strata declining about 10° from the perpendicular, toward the S. E., and forms a belt, stretching across the country from N. E. to S. W.

In a quarry where the argillaceous schistus, on the south side of this limestone belt, is worked for pavement, are impressions of what I think may safely be accounted marine objects; they have the form of shells, are much indurated, and full of pyrites. They seem to be of the same kind with the impressions said to be found in a slate quarry, near the village of Mat in Switzerland.[66]

[66] Hutton's Theory, vol. i. p. 327.

Another spot, affording instances of shells in primary limestone, is in Devonshire. On the sea shore on the east side of Plymouth Dock, opposite to Stonehouse, I found a specimen of schistose micaceous limestone, containing a shell of the bivalve kind: it was struck off from the solid rock, and cannot possibly be considered as an adventitious fossil.

Now, no rocks can be more decided primary than those about Plymouth. They consist of calcareous strata, in the form either of marble or micaceous limestone, alternating with varieties of the same schistus, which prevails through Cornwall to the west, and extends eastward into Dartmoor, and on the sea-coast, as far as the Berry-head. These all intersect the horizontal plane, in a line from east to west nearly; they are very erect, those at Plymouth being elevated to the north.

Though, therefore, the remains of marine animals are not frequent among the primary rocks, they are not excluded from them; and hence the existence of shell-fish and zoophytes, is clearly proved to be anterior to the formation even of those parts of the present land which are justly accounted the most ancient.

153. The rocks which contain sand or gravel, which are of a granulated texture, must also be considered as carrying in themselves a testimony of the most unequivocal kind, of their being derived from the detritus and waste of former rocks. Now, the fact stated in the text, concerning sand found in schistus, most justly accounted primary, might be exemplified by actual reference to many spots on the earth's surface. A few such will be sufficient in this place.

St Gothard is a central point, in one of the greatest tracts of primary mountains on the face of the earth, yet arenaceous strata are found in its vicinity. Between Ayrolo and the Hospice of St Gothard, Saussure found a rock, composed of an arenaceous or granular paste, including in it hornblende and garnets. He is somewhat unwilling to give the name gres to this stone, which M. Besson had done; but he nevertheless describes it as having a granulated structure.[67]

[67] Voyages aux Alpes, tom. iv. § 1822.

Among the most indurated rocks that compose the mountains of this island, many are arenaceous. Thus, on the western coast of Scotland, the great body of high and rugged mountains on the shores of Arafaig, &c. from Ardnamurchan to Glenelg, consists, in a great measure, of a granitic sandstone, in vertical beds. This stone sometimes occupies great tracts; at other times it is alternated with the micaceous, or other varieties of primary schistus; it occurs, likewise, in several of the islands, and is a fossil which we hardly find described or named by the writers on mineralogy. Much, also, of a highly indurated, but granulated quartz, is found in several places in Scotland, in beds or strata, alternated with the common schistus of the mountains. Remarkable instances of this may be seen on the north side of the ferry of Balachulish, and again on the sea-shore at Cullen. At the latter, the strata are remarkably regular, alternating with different species of schistus. At the former, the quartz is so pure, that the stone has been mistaken for marble.

These examples are perhaps sufficient; but I must add, that in the micaceous and talcose schisti themselves, thin layers of sand are often found, interposed between the layers of mica or talc. I have a specimen, from the summit of one of the highest of the Grampian mountains, where the thin plates, of a talcky or asbestine substance, are separated by layers of a very fine quartzy sand, not much consolidated.

The mountain from which it was brought, consists of vertical strata, much intersected by quartz veins. It is impossible to doubt, in this instance, that the thin plates of the one substance, and the small grains of the other, were deposited together at the bottom of the sea, and that they were alike produced from the degradation of rocks, more ancient than any which now exist.

154. In the Neptunian system, as improved by Werner, an attempt is made to take off the force of such instances as are produced in § 8, 9, and 152, &c. by distinguishing rocks, as to their formation, into three different orders, the primitive, the intermediate, and the secondary, or, to speak more properly, into primary, secondary, and tertiary. The same mineralogist distinguishes, among the materials of these rocks, between what he terms chemical and mechanical deposits. By mechanical deposits, are understood sand, gravel, and whatever bears the mark of fracture and attrition; by chemical deposits, those which are regularly crystallized, or which have a tendency to crystallization, and in which the action of mechanical causes cannot be traced. This distinction is founded in nature, and proceeds on real and palpable differences; but the application made of it to the three kinds of strata just enumerated, seems by no means entitled to the same praise.

The primitive rocks contain, it is said, none but chemical deposits, and are entirely composed of them: the intermediate contain a mixture of both, and also some vestiges of organized bodies: the secondary consist almost entirely of the mechanical, or of the remains of such bodies, with little of the chemical. The first of these, then, are held to contain no mark or vestige whatsoever of any thing more ancient than themselves, and are, in the strictest sense, primeval, or formed of the first materials, deposited by the immense ocean which originally encompassed the globe.

After them were formed the intermediate, mostly consisting of chemical deposits, but containing also some animal remains, and some spoils from the land, subjected to the various kinds of destruction, which even then made a part of the order of nature. These rocks, it is alleged, are chiefly argillaceous, are less indurated than the primary, and not intersected by veins of quartz.

The secondary were formed from the remains of the other two, and contain more mechanical deposits than any other.

This sketch of what I understand to be Werner's opinion concerning the different formation of the strata, is chiefly taken from a view of his system, in the Journal de Physique for 1800.

155. The main objection to the distinction here made between the primary and the intermediate strata, is founded on the facts that have been just stated. The sandstone of St Gothard is from a country having every character of a primary one in the highest perfection. The instances I have mentioned from the Highlands of Scotland, are from mountains, less elevated indeed than the Alps, but where the rock is micaceous, talcose, or siliceous, in planes erect to the horizon, and intersected by veins' of quartz. The shells from Plymouth are from a rock, that Werner would, I think, admit to be truly primitive. Those from the lakes, also, are from the centre of a country, occupied by porphyry, schorl, hornstone-schistus, and many others, about the order of which there can be no dispute. It is true, that in this tract there are argillaceous strata, of the kind that might be accounted intermediate, were they not interposed among those that are certainly primary; and this very intermixture shows, how little foundation there is for the distinction attempted to be made between the formation of the one and of the other. If there is any principle in mineralogy, which may be considered as perfectly ascertained, it is, that rocks similarly stratified, and alternated with one another, are of the same formation.

Hence we conclude, that there is no order of strata yet known, that does not contain proofs of the existence of more ancient strata. We see nothing, in the strict sense, primitive. It must be understood, that what is here said has no reference to granite, which I do not consider as a stratified rock, and in which neither the remains of organized bodies, nor sand, have I believe been ever found; though some instances will be hereafter mentioned, where granite contains fragments of other stone, viz. of different kinds of primary schistus.

To the instances of sand involved in primary schistus, I might have added many from the rocks of that order on the coast of Berwickshire, of which mention is so often made in these Illustrations; but I wished to draw the evidence from those rocks that are most unequivocally primary, and to which the Wernerian distinction of intermediate could not possibly be applied.

If any one assert, as M. De Luc has done, that sand is a chemical deposit, a certain mode of crystallization which quartz sometimes assumes, let him draw the line which separates sand from gravel; and let him explain why quartz, in the form of sand, is not found in mineral veins, in granite, nor in basaltes, that is, in none of the situations where the appearances of crystallization are most general and best ascertained.