BASAL CAMBRIAN ROCKS OF SHROPSHIRE. oltD 



these, they must have come from some mass of quartz now hidden 

 from view. jS'ow we know that the Monian rocks of Anglesey and 

 Ireland are richly provided with masses of quartz ; and these 

 abundant quartz-pebbles may indicate, I would suggest, the presence 

 of similar masses in the neighbourhood, now hidden by later deposits. 

 With regard to the remaining 5 per cent., Dr. Callaway has de- 

 scribed two, one a devitrified rhyolite, and the other a pcrlitic quartz- 

 felsite. I have examined thirteen of these pebbles, in fact an 

 example of every variety which I could recognize as distinct in 

 the field, in hopes of finding amongst them some of the underlying 

 greywackes or slates. Most of them were so much altered before 

 they were imbedded that they are now difficult to recognize. The 

 first group consists of granitoid and gneissoid rocks ; a specimen 

 from Lyth Hill is a typical gneiss,, apparently resulting from the 

 deformation of a granite ; another, from Pole Bank, is a typical 

 mica-schist, also very much pressed, and showing everywhere micro- 

 spectral polarization as in some of the more altered schists of Angle- 

 sey ; a third, also from Pole Bank, is a holocrystalline rock of large 

 elements, of which only quartz and felspar are now recognizable, 

 though there are dark patches which from their shape suggest a 

 derivation from mica. It is brecciated in parts and generally calls to 

 mind some of the more altered granites of Anglesey or the eurite 

 of the Wrekin. 



A second group consists of felsitic or rhyolitic rocks. One is a 

 very typical rhyolite, showing flow-structure to perfection. Another 

 is darker in aspect, but is full of well-marked spherulites, and its 

 insets are mostly of felspar. A third is also obscurely sphenditic, 

 but the secondary crystallization is carried so far as to make it 

 almost a macrofelsite. A fourth has decayed spherulites of irregular 

 outline now marked by irregular radiating lines of dark dust, and 

 between them are numerous irregular cavities as in an amygdaloid, 

 now filled with radiating zeolites (?). A fifth is nearly black, and 

 the numerous spherulites are almost entirely obscured by the 

 abundant dark amorphous dust which colours the rock. Different 

 as these rocks are to the naked eye, their insets in every case 

 declare them to be essentially quartz-felsites which had suffered 

 much alteration before they were imbedded. 



A third group consists of the doubtful rocks, whose external aspect 

 suggests that they are bedded ; one of these is a fairly coarse quartz- 

 felspar grit, obviously derived from a granitic rock, and which may 

 in fact be a fragment of greywacke. A second is a finer-grained, 

 more quartzose grit full of dust, and with nothing to suggest its con- 

 nection with a rhyolite. The other three are very plainly banded : 

 one, which to the naked eye looks exactly like a slate, has broad 

 parallel bands of dark amorphons dust, the clearer spaces being ex- 

 cessively fine polarizing material, which resembles very closely the 

 material of a slate from near All Stretton. There are, however, in 

 this rock several large isolated fragments of quartz and felspar which 

 resemble insets, and it is possible therefore Ave have here only a 

 banded ash. Another also contains irregular bands of dust, and 



