Evidence of contemporaneous volcanic action in the Banffshire schists / by William Mackie.

  • Mackie, William.
Date:
[1906]
Catalogue details

Licence: In copyright

Credit: Evidence of contemporaneous volcanic action in the Banffshire schists / by William Mackie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

    9/19 (page 97)
    3 feet across if as much, this is succeeded by a crumbling rock which breaks down into rough nodules in which it is difficult to obtain a fresh fracture. It is a dark, micaceous, schistose rock, showing numerous white lenticles, which are somewhat local in their distribution, but in the patches where they do occur are often seen in close set aggregates. A micro-section of this rock is submitted. It shows lenticles of granular quartz and a micaceous substance, the latter in sparing quantity, set in a matrix of fine biotites and felspars, and is exactly like one of the specimens of No. 4, except that the lenticles are of quartz instead of calcite. Whether a variety exists, as in the case of that rock, showing hornblende in addition, can only be settled by examining a number of micro-sections from this particular outcrop. Following this band on the west comes a band of dark quartzite similar to No. 3, and this is faced on its western side by a band of dark frilled schist, which is separated by a large fissure marking the line of a fault from a dark, finely crystalline hornblendic rock, closely resembling a dolerite. This forms the centre of fold hi., and as all the types of the volcanic rocks have now come under observa- tion in the section just described, it will be unnecessary to follow the rock sequence farther along the course of the river. As will be noted, four types of volcanic rocks have been seen. These briefly are :— Type A, represented by No. 2 Compact Vesicular, cavities empty or filled with CaCog. Type B, represented by Nos. 4, 8, and 8a, Pumiceous Vesicular with CaCog. Type C, represented by Nos. 5 (?) and 10, “ Nodular Igneous,” Compact Hornblendic Beds. Type D, represented by No. 11, Micaceous Vesicular, cavities with quartz-mica aggregates. I shall now trace the development of these rocks in the sur- rounding area in so far as we have indications of their presence in the various limited exposures that exist. Type A.—Indications of A occur in the direct line of strike on the top of the Poorman, the point at which vesicular volcanics associated with the Banffshire schists were first observed. On following the line of strike still farther to the S.E., blocks'of an exactly similar rock were found in a small surface opening on the moor overlooking the hamlet of Ardivalloch, about J of a mile N.E. of a small hill-croft locally known as Mount Pisgah. Again, in the line of strike to the N.E. of our central section, the presence of similar rocks is indicated by the presence of surface blocks of similar character on the hillside behind the U.F. Church at the Lower Cabrach ; while a series of beds, which