BASAL CAMBRIAN ROCKS OF SHROBSHIRE. 417 



the Longmynd rocks are derived from, and therefore later than, the 

 Uriconian, will now only hold good for the upper of its two series. 



We shall find it best, in considering- what must be the result of 

 the present observations, to commence at the top of the succession. 

 The lowest beds that are continuous throughout the area, i. e. those 

 found on both sides of the Longmynd hills, are the Shineton shales, 

 and these have been determined to belong to the Upper Cambrian. 

 Below these on the east there is the Comley sandstone and quartzite, 

 enclosing a fauna which does not elsewhere characterize the beds 

 immediately below the Upper Cambrian, but which forms the lowest 

 fossil group of that system. Whether the Cambrian series is here 

 complete and the intervening fauna lies yet hidden in the rocks, 

 whether there is a yet unrecognized break between them, or whether 

 such intervening fauna ever existed, it is not for me to decide. It 

 is certain, however, from what is known elsewhere, that we cannot 

 expect to discover here any truly Cambrian fauna below that of the 

 Comley sandstone. On this side, therefore, of the Longmynd Hills 

 we are at the base of the ordinary Cambrian development. On 

 the other side we find an enormous thickness of grits and slates 

 bearing no resemblance to the quartzite * and totally devoid of 

 fossils. These grits and slates, though doubtless deposited with 

 much greater rapidity than the rocks on the eastern side, must yet 

 have required an enormous length of time for their formation, and 

 starting at the top in Upper Cambrian, the base must belong to a 

 very early, if not the earliest possible, portion of that series. The 

 series must therefore, to some extent, be synchronous with, and to 

 some extent earlier than, the quartzite, and have been formed in a 

 different basin. 



It is to be regretted that the relations of the two groups of rocks 

 to one another are not as yet indubitably settled. Nevertheless such 

 indications as there are, and which have been enumerated above, 

 point with some force to the conclusion that the lower part, at 

 least, is anterior to the quartzite, and is therefore the oldest repre- 

 sentative of the stratified Cambrian rocks in the whole district. For 

 these rocks, inasmuch as they still form the bulk of the Longmynd 

 Hills, whose highest summits they actually occu])y, the name of the 

 Longmynd series can still be retained, and this series will therefore, 

 in this district, be the base of the Cambrian. 



On the eastern side of these hills, however, we must now recog- 

 nize a comparatively narrow band, which belongs to an older series. 



It is this band, and not the conglomerates, grits, and western 

 band of slates, which has been studied by Salter and in which he 

 discovered organisms totally unlike, so far as they go, the fauna of 

 the Cambrian. It is this that shows a resemblance to the rocks of 

 Bray Head, containing somewhat similar organisms, and it is this 

 that has been compared with the schists of 8. Lo, which are admitted 

 to be unconformably overlain by the " conglomerat i)ourpre '' 



* The only exception to tliis is a roof of quartz running fairly parallel 

 to the bedding at " Roch," near.Medlicott. in the midst of the purple slates; 

 but this appears to be rather ol" the nature ol a \oin. 



