Transactions and Proceedings

APPENDIX II.

NOTE ON ROCK-JUNCTIONS IN THE DISTRICT.

The alteration of rocks through the intrusion of molten or heated matter is a subject of special interest to the geologist, as affording, among other things, conclusive evidence as to the relative age of the rocks concerned. The following examples occurring in the district may be mentioned :

1. Junction of basalt with sandstone, quarry near village of
Monzie, Crieff. Interesting example of a sill.

2. Fintalich Quarry, avenue (east of road), Drummond Castle,
Muthill. Alteration of sandstone by basaltic intrusion.
3. Junction of basalt and greywacké in whinstone quarry on hill
north of Comrie.

4. Road cutting (and railway cutting near by), between Comrie
and St. Fillans. Narrow basalt dyke in greywacké.

5. Alteration of eurite by heat of basalt intrusion. Top of Glen Mathaig. Eurite takes on flinty texture.

II. The Amaboid Agates of Monzie, near Crieff.

By WALTER KERR, M.A.

(Read 12th November, 1903.)

For the sake of clearness we must have recourse to definition. Much confusion attaches to the use of one of the terms placed at the head of this paper. In many books on mineralogy, including even recent ones, the agate is considered as a mineral, and appears in lists of minerals. It is defined as a mineral, and treated as of definite form and constant composition. I venture to submit that some change in the use of the term is desirable, a change in the direction of greater exactness and at the same time of wider range.

For the purposes of this paper, an agate may be defined as a particular rock structure consisting of a segregated mineral or minerals arranged in layers, some form of quartz, either crystalline or colloid, being present as the main constituent. It will be fairly obvious, after study of the Monzie specimens, that the term agate needs some such revision as suggested, unless these specimens are to be excluded from the category. I feel sure, however, that few geologists will, after consideration, doubt that these nodules are

true agates. If the agate is regarded as "a banded chalcedony," as it is sometimes defined, we at once see, on application of the term to many of the fine Montrose specimens, that the outer layer of celedonite, as well as the inner layers of crystalline quartz, oxide of iron, or opal, must be left out of account. The reason for retaining crystalline layers as part of the agate itself is greatly strengthened when we carefully observe the Monzie specimens, for some of these specimens contain no chalcedony at all, and yet they are composed of at least three well-defined layers of segregated minerals.

LOCALITY.

The place where these amoeboid agates are found is a small area in the bed of a stream which joins the Shaggie on its left bank about a quarter-mile above the village of Monzie. The exact locality of the specimens is immediately below the point where the old road between Hosh and the Sma' Glen crosses the stream. Published geological maps place the boundary between sandstone and porphyrite some distance to the west of the locality indicated, but, as a matter of fact, a considerable area marked on the maps as lower old red sandstone is igneous rock, which, for the most part, is covered with glacial drift. The agates are found at a place where the little stream has cut through the drift and exposed the porphyrite.

ASSOCIATION.

The rock at this place is not nearly so uniform and compact as at other points of outcrop to the south-west. Some fifty yards down the stream we find a vein of barytes, about nine inches in thickness, which is cut through by the stream. Proceeding up stream, porphyrite is met with, containing small amygdules of calcite, quartz, and chalcedony, and of quite the usual egg-like form. Still higher up, we find the larger agates under consideration, characterised by the distinctive form for which I have suggested the term amoeboid, on account of the remarkable resemblance of their projections to the pseudopodia of the amoeba.

Further up the stream, and east of the road referred to, the mineral inclusions of the rock assume the form of jasper veins, running in various directions. Probably the reef of barytes already mentioned continues beyond this point, as I have found loose pieces of this mineral in the bed of the burn at the place.

SIZE AND FORM OF THE AGATES.

The size of the specimens met with ranges from one to six inches in longer diameter. In general shape they are oval nodules, but with

a highly irregular surface, owing to the projections mentioned. When the outer layers have been weathered away, or artificially dissolved by hydrochloric acid, the remaining body presents a very striking appearance. Hollow spherical depressions lie between the projections, while the complete surface itself, including the projections, offers one of the most interesting examples of natural mould formation. The exact reverse of a crystalline surface of rhombic form is presented to view.

COMPOSITION AND ARRANGEMENT OF PARTS.

As already indicated, the nodules consist of well-defined mineral layers. These are:-Oxide of iron (probably turgite), dolomite (in the free crystalline form of pearl spar), carnelian, cacholong or other opal, and rock crystal. Some variety in arrangement of these layers is observed, but the oxide of iron (with platey structure) is always the outermost, i.e., next the rock wall; and the pearl spar always forms a single layer superimposed on the turgite. In the remaining portion of the nodule, i.e., that within the mould of pearl spar, we notice the following varieties:

1. Solid unbanded carnelian, with or without a hollow centre. In the former case the free surface of the carnelian has the usual botryoidal form. Often the hollow is found

filled with secondary calcite.

2. Solid crystalline quartz.

3. Solid calcite.

4. Solid finely banded carnelian, with no cavity.

5. Broad band or bands of carnelian next the dolomite, with quartz heart. This is the commonest variety.

6. Solid finely banded carnelian or chalcedony, filling the heart of the nodule, and surrounded by a layer of crystalline quartz, which again is separated from the dolomite by a layer of carnelian.

7. Within the pearl spar a layer or layers of carnelian, succeeded by a single thin band of cacholong, enclosing a quartz heart.

8. A layer of carnelian succeeded by alternating bands of cacholong and chalcedony surrounding a heart of quartz or calcite.

9. Alternate bands of carnelian and crystalline quartz (repeated up to four times), with quartz heart.

10. Like (5), but with central heart of carnelian.

In consequence of the action of weathering agents, water and air, the layer of pearl spar is sometimes dissolved away and the

layer of oxide of iron reduced to a purplish-red ochre. Occasionally one is fortunate enough to find the jacket of pearl spar left in situ, while the body of the nodule has been removed. The well-known botryoidal or knob-like form of the pearl spar is then seen. It is this structure which gives rise to the spherical depressions observed on the surface of the inner core already mentioned.

NEGATIVE FEATURES.

Certain negative features of interest may be observed in connection with the Monzie agates :

1. They have no celedonite or other talcose mineral associated with them.

2. No suggestion of stalactitic structure appears in them.

3. Onyx structure is entirely absent.

4. Other gravitational effects, such as unequal thickness of layers, do not seem to be exhibited.

5. No definite tube of escape has been noticed in them.

6. Disk formation, eye structure, faulting of bands are not met

with.

ORIGIN.

I venture to suggest that the amoeboid agates of Monzie are probably different in their origin from ordinary agates. The current conception of a simple vapour cavity will scarcely account for their highly irregular form. No doubt a vapour cavity in a viscous lava in motion, and subject to pressure, may assume an elongated or flattened form, as observed in the common agate, but some subsequent change, such as the partial bulging in of the rock walls, seems necessary to the production of the cavity in the case of the Monzie agates. It is worthy of notice that it is not only the core within the dolomite layer that exhibits the irregular form referred to, but this form characterises the whole nodule within the rock walls. Further, it seems to me probable that the substances which contributed the different layers— iron oxide, dolomite, carnelian, etc.-existed together at one and the same time in solution within the cavity, and then as changes of temperature and pressure took place, successive deposit was effected from within, instead of from without, and through the layers already formed as in ordinary agates. I would suggest that the fact of the outer layers of the nodule being crystalline, instead of colloidal, points to an origin different in essential features from that of other agates. At any rate, the unique features of highly irregular form and outer crystalline layers observed in the Monzie specimens cannot fail to afford special interest to those who have given attention to the study of mineral segregation.

III.-Preliminary List of Minerals occurring in Perthshire.