Fig. 3.
False-bedding on a large scale may be a cause of error. In the Penrith Sandstone of Cumberland, the planes of deposition are often found dipping in one direction in a large quarry, but inspection of a wider area shows that this is not the true dip of the beds as a whole, but merely a local dip due to deposition on a slope, and any one attempting to calculate the total thickness of the beds by reference to these divisional planes might be seriously led astray. A reference to [Fig. 4] will explain this. The lines AA´, BB´ are the true bedding-planes cut across in the section, whilst the lines sloping to the right from xx are only lines of false-bedding on a large scale. An exaggerated estimate of the thickness of the deposit would be made by measuring the thickness of each of these stratula from A to A´ and adding these thicknesses together, whereas the actual thickness of the middle bed is the distance between A and B or A´ and B´.
Fig. 4.
When rocks have been affected by thrust-planes, the simulation of bedding may be carried out to a very full extent. Not only do the major thrust-planes resemble bedding-planes but the minor thrusts produce an appearance of divisional planes separating stratula or laminæ, and a close approximation to false-bedding is the result. To this structure Prof. Bonney has given the name 'pseudo-stromatism[17].' It may be developed in rocks of all kinds, whether possessing original planes of stratification or not, and as a result of its existence the geologist may be seriously misled, not merely by mistaking the direction of the strata, but also the nature of the rock, for we may find it produced in an unstratified glacial till, and in a massive igneous rock, and in each case the resulting rock will resemble a sedimentary deposit, and of course the observer may be confirmed in his erroneous opinion by the formation of apparent fossils, ripple-marks or other objects which he might expect to discover in sediments. As illustrative examples, reference may be made to a number of schistose rocks, in which the planes of discontinuity (which are in truth planes of foliation) have been taken for bedding-planes and the rocks claimed as sedimentary though they are in reality igneous; for instance many of the rocks of the Laurentian of Canada, of the Hebridean of the North West Highlands, and some of the ancient rocks of Anglesey.
[17] Bonney, T. G., Quart. Journ. Geol. Soc., vol. XLII. Proc. p. 65.
A foliated structure may, as is now well known, be simulated by a structure developed in a rock prior to its consolidation. The similarity of flow structure of some lavas to the foliated structure of a schist was long ago pointed out by Darwin and Scrope, and recent work has proved that parallel structure due to differential movement prior to consolidation may be developed in plutonic rocks, as shown by Lieut.-General McMahon in the Himalayan granites, and by Lawson amongst the plutonic rocks of the Rainy Lake Region; and as the foliated structure may be mistaken for original stratification the same may occur, and has occurred, when dealing with this flow-structure.