Mr. Watts has further observed that the rocks containing no olivine offer greater difficulties in classification than those in which that mineral is present. "The very distinction," he remarks, "between dolerites and basalts is less marked, the types are much less sharply distinguished, and decomposition and masking of the structure are more common. While using the term Dolerite for such rocks as have a sub-ophitic structure, I have extended it to those rocks in which evidence exists that a great part of the crystallization took place under intratelluric conditions. Although not quite holocrystalline, the crystals of felspar, augite and magnetite are large and the structure coarse-grained, while the groundmass is confined to comparatively small interstitial patches. In these rocks there is usually no one dominant porphyritic ingredient."
| I. The Olivine-bearing Series | |||
| 1. Olivine-Dolerites | |||
| 1a. | Porphyritic elements inconspicuous, olivine being the principal, and felspar of secondary importance; groundmass sub-ophitic. | } | Jedburgh Type. |
| 1b. | Strongly porphyritic; felspar-phenocrysts large; olivine smaller; groundmass sub-ophitic. | } | Kilsyth Type. |
| 1c. | Porphyritic olivine, but not felspar; groundmass sub-ophitic. | } | Gallaston Type. |
| 2. Olivine-Basalts | |||
| 2a. | Porphyritic olivine, augite and felspar; groundmass of felspar-laths imbedded in granules of augite. | } | Lion's Haunch Type. (See [Fig. 207].) |
| 2b. | Porphyritic olivine and augite; groundmass of felspar-laths imbedded in granules of augite. More rarely the groundmass is made of idiomorphic augite imbedded in felspar-substance. | } | Craiglockhart Type. |
| 2c. | Porphyritic olivine abundant, augite much less common, and felspar very rare or absent; groundmass with granular or idiomorphic augite (one of the most common types). | } | Dalmeny Type. |
| 2d. | Porphyritic olivine more common than augite; groundmass of granules of augite set amongst lath-like felspars which are much fewer in number than in 2c. | } | Picrite Type. |
| 2e. | Porphyritic olivine more common than augite; groundmass of idiomorphic augite imbedded in felspathic material which is not abundant. | } | Limburgite Type. |
| 2f. | Porphyritic olivine and felspar, without augite; groundmass of granular or idiomorphic augite, with lath-shaped felspars. | } | Kippie Law Type. (For analysis see [p. 379]). |
| II. The Non-olivine-bearing Series | |||
| 3. Olivine-free Dolerites | |||
| Felspar, augite, magnetite in coarse-grained aggregate usually ophitic or sub-ophitic; groundmass not plentiful. | |||
| 3a. | Groundmass absent | Ophitic Type. | |
| 3b. | Groundmass micropegmatitic | Ratho Type. | |
| 3c. | Groundmass an unstriated felspar (not orthoclase) and occasionally some interstitial altered glass or a little quartz. | } | Burntisland Sill Type. |
| 4. Doleritic Basalts | |||
| Felspar, augite and magnetite in coarse-grained aggregate; groundmass rather more plentiful and often in large patches. | |||
| 4a. | Felspar and augite, related sub-ophitically where together, but augite showing crystalline contours in contact with the groundmass; some interstitial quartz and unstriated felspar. | } | Bowden Hill Type. |
| 5. Basalts | |||
| Finer-grained rocks, generally with a porphyritic ingredient and much scattered interstitial matter in the groundmass. | |||
| 5a. | Porphyritic felspar, and occasionally a little augite; groundmass of granular augite, felspar needles and magnetite with some interstitial matter. | } | Binny Craig Type. |
| 5b. | Porphyritic felspars not conspicuous and small; the rock mainly made up of a mesh of fine felspar-laths set amongst granular augite, magnetite and base. | } | Tholeiite Type. |
| 5c. | Similar to the last but even finer-grained, and with the } - base in a cryptocrystalline condition. | } | Cryptocrystalline Type. |
Taking first the superficial lavas, I know of only one locality where picrite occurs in such a position that it may be included among the surface outflows. This is the quarry near Blackburn, to the east of Bathgate, where I originally observed it.[445] The rock occurs there on the line of the basalt-flows from the Bathgate Hills, and I mapped it as one of them before the microscope revealed the remarkable composition of the mass. I still believe it to be a lava like the "leckstone" described on [p. 443], though the other known examples of this rock in the basin of the Firth of Forth are intrusive sheets. The rock locally known as "leckstone" or "lakestone" has long been quarried for the purpose of constructing the soles of bakers' ovens, as it stands a considerable temperature without cracking. Its microscopic structure is now well known. As exposed in Blackburn quarry, an interesting difference is observable between the lower and upper parts of the sheet. The lower portion is a picrite, with abundant serpentinized olivine, large crystals of augite, and a considerable amount of ores. The upper portion, on the other hand, has plagioclase as its most abundant definite mineral, with a minor quantity of minute prisms of augite and of iron-ores, and scattered crystals of olivine. Here, within the compass of a few yards and in one continuous mass of rock, we have a transition from a variety of olivine-basalt into a picrite.
[445] Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 506.
The great majority of the puy lavas belong to the olivine-bearing series. A few of them are dolerites, but most are true basalts of the Dalmeny type, of which typical examples may be seen at the Kirkton quarries, Bathgate, and in the coast section between Pettycur and Kinghorn. Occasionally they present transitions towards picrite, as in the sheet overlying the lowest limestone at Kirkton, and in the lowest lava of King Alexander's Crag, Burntisland. These puy lavas exhibit considerable variety of structure as seen in the field. Some are solid, compact, black rocks, not infrequently columnar and weathering into spheroidal exfoliating forms. Others are somewhat granular in texture, acquiring green and brown tints by weathering, often showing amygdaloidal kernels, and even passing into well-marked amygdaloids. Many of them exhibit a slaggy structure at their upper and under surfaces (Figs. [153], [170], [171]). These external differences are an index to the corresponding variations in composition and microscopic structure enumerated in the foregoing tabular arrangement.
As a rule, the basic rocks which occur intrusively in connection with the puys, especially where they form a considerable mass, have assumed a much more coarsely crystalline texture than those of similar composition which have been poured out at the surface. They are generally dolerites rather than basalts. But with this obvious distinction, the two groups have so much in common, that the geologist who passes from the study of the subterranean phenomena of the Plateaux to that of the corresponding phenomena of the Puys is at once impressed with the close relationship between the material which, in the case of the puys, has consolidated above ground, and that which has been injected below. There is no such contrast between them, for example, as that between the basic and intermediate lavas of the plateaux and the more acid intrusions associated with them.
By far the largest number of the basic sills, bosses and dykes associated with the puys are somewhat coarsely crystalline dolerites without olivine. They include, however, olivine-dolerites and basalts, and even some extremely basic compounds. Of these last, a typical example is supplied by the now well-known picrite of Inchcolm, in the Firth of Forth, which occurs as an intrusive sheet among the Lower Carboniferous Sandstones.[446] In recent years one or two other picrite-sills have been observed in the same district. An interesting example has been described from a railway cutting between Edinburgh and Cramond where the rock invades and alters shales. More detailed reference to it will be made in the account of the sills connected with the puys. Another instance of the occurrence of this rock is in a railway cutting immediately to the west of Burntisland where it has been intruded among the Calciferous Sandstones below the Burdiehouse Limestone.
[446] Trans. Roy. Soc. Edin. vol. xxix. (1879) p. 506. Teall, British Petrography, p. 94.
Rocks approaching limburgite occur among the sills and bosses which pierce the Carboniferous Limestone series of Fife between Cowdenbeath and Inverkeithing. One of these is found at Pitandrew, near Fordel Castle. Dr. Hatch observed that it consists of "numerous porphyritic crystals of olivine, with a few grains of augite and an occasional small lath-shaped crystal of felspar imbedded in a groundmass which is composed principally of idiomorphic augite microlites, small crystals of a brown mica, granules of magnetite and prisms of apatite. In addition, there is a considerable amount of interstitial matter, which is partly colourless glass, and partly shows a slight reaction between crossed nicols." Another example of the same type of rock occurs as a plug or boss in the tuff-vent of the Hill of Beath, and a further display of the limburgite type is to be seen in Dunearn Hill near Burntisland.
Although olivine-basalts of the Dalmeny type are most frequently met with as interstratified lavas, they also occur as bosses and sills. The typical example from Dalmeny is itself intrusive. Other illustrations are to be found in the Castle Rock of Edinburgh and in the sheets near Crossgates and Blairadam in Fife. The presence or absence of olivine, however, may sometimes be a mere accident of cooling or otherwise. I have shown that in the same mass of rock at Blackburn a gradation can be traced from a rock largely composed of altered olivine into one consisting mainly of felspar with but little olivine, and another example occurs in the picrite-sill between Edinburgh and Cramond. Dr. Stecher has ascertained that the marginal portions of the sills in the basin of the Firth of Forth, which cooled first and rapidly, and may be taken, therefore, to indicate the mineral composition of the rock at the time of extrusion, are often rich in olivine, while that mineral may be hardly or not at all discernible in the main body of the rock.[447]