58 



HARDWICKE'S SCIENCE-GOSSIP. 



shall see that in few or no points did he miss the 

 mark. 



From several points of view these rocks are of great 

 interest, for they admit us to a whole volume of 

 the earth's history, which, when carefully read and 

 thoroughly understood, will enable us (in part, at least) 

 to bridge the gulf between the knowledge we have 

 from stratigraphical geology on the one hand, and from 

 celestial physics and chemistry on the other. But this 

 volume is written in an obsolete language, and the ink 

 was blotted and smeared before it had thoroughly set 

 in ; and, in addition to this, the volume has been cut 

 and torn into fragments, of which many have been 

 entirely lost and many have become almost useless. 

 So the translation is difficult from a triple cause, for 

 we have to piece the rocks together and to supply 

 what is lost or missing, to see through the changes, 

 defacing, and obliteration which the original charac- 

 ters have undergone, and to reason out the methods by 

 which the rocks were formed ; for we cannot suppose 

 that the agencies at work at these distant 

 periods were precisely like those we can 

 observe to day. 



In spite of these difficulties, the Pre- 

 Cambrian rocks are being gradually re- 

 duced to order, just as the chaos of "Tran- 

 sition and Greywacke " grew up, under the 

 skilled workmanship of Sedgwick and Mur- 

 chison, into the beautiful and connected 

 Cambrian and Silurian systems. 



In America, Dr. Sterry Hunt has identi- 

 fied six distinct systems below the Potsdam 

 sandstone ; they are, in ascending order : — 



I, Laurentian ; 2, Norian ; 3, Huronian; 

 4, Montalbian ; 5, Taconic ; 6, Keweenian. 



Of the first four of these an excellent 

 account was published in Science-Gossip 

 some time back. Each of these systems 

 contains a great thickness of rocks, but they cannot 

 be definitely correlated with their equivalents on this 

 side of the Atlantic. 



Neglecting the American, Scotch, and Irish Pre- 

 Cambrians, I propose to give a resume of the characters 

 of these rocks in the district where they occur in 

 England and Wales ; there are seven such districts, 

 and, in the order in which they were originally 

 described they are, 1, Malvern (1S64. Holl, Q. J. 

 G. S. xxi.) ; 2, St. David's (1876. Hicks, Q. J. G. S. 

 xxxiii. &c.) ; 3, Caernarvonshire and N. Wales (1877, 

 Hicks, Bonney, Hughes, Q. J. G. S. xxxiv. xxxv. 

 &c); 4, Anglesea (1877. Hicks, Hughes, Callaway, 

 Roberts, Q. J. G. S. xxxiv. xxxvi. &c; Geol. Mag. 

 1880, 1SS1); 5, Shropshire (1879. Callaway, Q. J. G. S. 

 xxxv. &c.) ; 6, Charnwood Forest (1876. Bonney and 

 Hill, Q. J. G. S. xxxiii. &c.) ; 7, '.Cornwall. 



1. Malvern. Holl, in 1864, wrote his splendid 

 paper on these hills, and boldly claimed a Pre- 

 Cambrian age for the gneisses there. -The rocks are 

 mostly felspathic and hornblendic gneisses, some- 



times massive, but more frequently well foliated. 

 Mica occurs plentifully in some parts, for instance 

 near the Wych, and epidote is a rather common 

 mineral. The fine hornblende rock of North Hill is 

 well known for its handsome appearance and remark- 

 ably tough character. There are good granitoid rocks 

 here and there, some closely resembling those of the 

 Wrekin. The general strike of these rocks is along 

 the chain, i.e. north and south, but there are many 

 local variations. The dip varies greatly, and is 

 frequently as much as 85°. 



The sketch (rig. 45) gives an idea of the character 

 of the bedding of the rock, which agrees with its 

 foliation. 



Intrusive granites and diorites occur all along the 

 ridge, and some care is requisite to distinguish be- 

 tween the igneous and metamorphic rocks. On the 

 east side of Herefordshire Beacon there is a curious 

 halleflinta-like rock, which Dr. Callaway classes as 

 Pebidian. Although the strike of the Pre-Cambrian 



Fig. 45 

 rock 



— Knoll south of Worcester Beacon. 1, Compact Hornblendic 

 ; 2, coarse gneiss, with lenticular patches of fine gneiss in it. 



rocks agrees with that of the surrounding Cambrians, 

 the dip is often in an entirely opposite direction, and 

 in Raggedstone Hill, Holl has given a section of the 

 unconformity splendidly displayed (fig. 46). As the 

 Hollybush sandstones are probably the equivalents 

 of part of the Lingula flags, there cannot be any 

 doubt as to the Pre-Cambrian age of the gneisses, 

 which are now acknowledged to belong to the Dime- 

 tian system. 



2. St. David's. About the same time as Holl was 

 working at the Malverns, Dr. Hicks announced the 

 presence of Pre-Cambrian rocks underlying the Cam- 

 brian at St. David's. He did not however begin to 

 work on these at once, but felt his way down to them, 

 by working out the Cambrian thoroughly. He esta- 

 blished the succession in descending order through 

 Arenig, Llandeilo, Tremadoc, Lingula, and Menevian 

 beds, down to the Harlech group in which he dis- 

 covered a fauna of Trilobites, Brachiopods, and 

 Annelids, exceedingly rich for beds so low in the 

 series. Below these rocks were ashy shales, and 



