Trof. W. Keeping — Geology of Ahenjstwyth. 537 



for this is the grand order of English rock sti-ata, that the beds to 

 the west slope under those to the east ; the beds of the east over- 

 lying those of the west. But again, it is the primal law of strati- 

 graphical geology that the undermost strata are the oldest ; and by 

 this law we learn that the rocks of Cambria are the lowest and 

 oldest of rocks — they are the foundation over which England has 

 been built up. The very lowest deposits of the Cambrian System 

 are, however, not to be found here at Aberystwyth, though they may 

 be readied and seen in grand development by a day's excursion to 

 Barmouth, Our local rocks are much newer than those of Bar- 

 mouth, and very probably belong to the lower part of the great 

 Bala Group of Prof. Sedgwick. By the Geological Survey they 

 were placed in the Lower Llandovery Group ; and there are 

 apparently conflicting facts which have yet to be put into harmony. 



The suite of fossils as recorded by Salter from the Devil's Bridge 

 has a Lower Llandovery facies, but other fossils, such as the 

 Dictyonema, the Dendroid Graptolites, and the Bast7-ites, seem to be 

 represented by similar, if not identical forms in the Llandeilo 

 Group of the Longmynd district; the curious Polyzoon (?), the 

 worm tracks and other markings, are near to those of the Skiddaw 

 Group in the English Lake District, and some similar markings are 

 also found in the Arenig country. Whilst recognizing then the 

 present imperfection of the evidence, we may reasonably place the 

 Aberystwyth rocks in the Lower Bala Group of Sedgwick.^ Litho- 

 logical resemblances afford us little help, but we may notice that 

 some of the Silurian rocks of North Wales (in the Denbighshire 

 Group) have a great likeness to those of Aberystwyth. 



Slates and Slaty Structures. — It is proper to notice here, in further 

 detail, those mineral masses which are so important as sources of 

 mineral wealth to our district. These are principally the slates and 

 metals as worked in slate quarries and metalliferous mines. 



Slaty structure has been superinduced upon clayey and other 

 rocks after their deposition by the action of lateral pressure. It is 

 reasonable to our common sense that when a number of lenticular 

 bodies are subjected to lateral pressure, and are allowed some slight 

 freedom of vertical motion, they will get more or less arranged face 

 to face, with their broad faces at right-angles to the pressure. So, 

 too, if we wished to subject a lot of pennies to heavy lateral pressure, 

 we should first arrange them in rows face to face, and apply the pres- 

 sure along the course of the rows, for in this arrangement they are best 

 able to sustain the pressure, and this arrangement is the most stable 

 under the circumstances. Just so it is with the old clay deposits. 

 We actually see under the microscope the particles of slate with 



1 N.B. — These rocks are very commonly known as Silurian. Through a series 

 of errors. Sir Eoderick Murchison felt compelled to absorb Sedgwick's Cambrian 

 almost entirely into his Silurian System. But so good and true a name as the 

 " Cambrian System," as applied to a great and natural group of rocks, honestly and 

 successfully worked out, must not be allowed to be degraded into the appellation of 

 a small part of that system. Such a com-se is not only unfair to our great leader in 

 Cambrian geology, but is also, as I believe, a hindi'ance to geological science. As a 

 name, too, the change would be unfortunate. 



