The Quarterly journal of the Geological Society of London . slate in the quarry, and if there be any grit it is notolder than the felsite. This is what I found. A greenstone dykehas been almost quarried out, and the felstone is left as a kind ofminiature cliff overlooking the valley. Part of the greenstone isshown in Mr. Blakes plan; but its northward prolongation shouldnot be represented as adjoining a band of grit, for it runs continu-ously in contact with the felsite ; and the greenstone should not belimited on the plan to the northern end of the quarry, for it can betraced, by means of sma
The Quarterly journal of the Geological Society of London . slate in the quarry, and if there be any grit it is notolder than the felsite. This is what I found. A greenstone dykehas been almost quarried out, and the felstone is left as a kind ofminiature cliff overlooking the valley. Part of the greenstone isshown in Mr. Blakes plan; but its northward prolongation shouldnot be represented as adjoining a band of grit, for it runs continu-ously in contact with the felsite ; and the greenstone should not belimited on the plan to the northern end of the quarry, for it can betraced, by means of small bosses clinging to the base of the scarpof felsite, southward as far as the road, and it is seen even high boundary-wall at this end is constructed mainly of green-stone blocks, doubtless obtained from the quarry, and two smallmasses still remain in situ built into the wall as buttresses * (figs. 2and 3). The greenstone normally is an ordinary, rather coarse Fig. 3.—Quarry at Bryn Efail; face of cliff seen from lower ground. G. Greenstone. dolerite, with ophitic structure, but it becomes fine-grained for thespace of about a foot from the felsite, and the band ^ inch widealong the boundary is a dirty sahlband, or an edge still partiallytachylytic. This greenstone (or diabase), in general structure andcomposition, agrees with those which are so common over all theNorth-Wales district; thus it would have been rather curious if it hadbelonged to a distinct and very much earlier period. The characterof the edge, however, proves beyond doubt that it is intrusive in thefelsite, so it may be of the same age as similar masses Mr. Blakes description of his slides appeared to be very minute,I was careful to collect the most dubious-looking specimens formicroscopic examination. I took most of these at intervals alongthe boundary of the felsite, searching for an example of the so-called * The greenstone may have extended to the edge of the ground
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Keywords: ., bookcentury1800, bookdecade1840, bookidquarte, booksubjectgeology
