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SCIENCE-GOSSIP. 



THE LOWER CAMBRIAN SERIES 



With some Suggestions. By Alexander Ramsay. 

 (Continued from page 53.) 



TN forming a stratigraphic grade scale the first 

 desideratum is clearly a continuous succession of 

 fossiliferous strata covering the whole of the period 

 and merging into the strata representing the ante- 

 cedent and succeeding periods; but these are just 

 the kind of beds which are least adapted for 

 furnishing base lines. Such base lines are best 

 formed by conglomerates, which are, as a rule, far 

 distant from the continuous series of beds used for 

 time-valuing the period ; this makes the task of 

 correlating the strata a very difficult one. It 

 often happens, too, that the available sections are 

 largely composed of limestones, the present thick- 

 ness of which may be very different from the 

 aggregate thickness of the original deposit, owing 

 to more or less of it having been dissolved away. 

 This error is partly remediable by carefully 

 measuring the separated insoluble matter when- 

 ever the origin of this can be traced to the solution 

 of the limestone. At present no section, so far as I 

 know, has been examined from this point of view, 

 so that the thicknesses and calculations here given 

 are adversely weighted with the errors due to this 

 cause. 



Owing to the fact that the Cambrian rocks and 

 Cambrian fossils of geologists are so intimately 

 linked together, and to the further fact that every- 

 where, or almost everywhere, such Cambrian rocks 

 are laid down unconformably upon older rocks, 

 it necessarily follows that there is a stratigraphical 

 break between the rocks generally assigned to the 

 Cambrian and Huronian periods respectively. In 

 order to free myself from this vicious circle of 

 argument I am compelled to cut through it some- 

 where, and this I do by taking the zone of Para- 

 doxides as a provisional time-horizon and measuring 

 from it upwardly and downwardly. The zone 

 of Paradoxides, in Pembrokeshire, is called the 

 Menevian group of strata, and its highest bed is 

 here provisionally calculated to be the commence- 

 ment of eon 113, or, in other words, the end of it 

 nearest to the present time. It is also supposed 

 that the Paradoxides beds of the St. John's group, 

 in New Brunswick, is of the same date. It is 

 further assumed that the L. Cambrian of the 

 typical Cambrian area is represented by the strata 

 corresponding to the eons 116 to 107, both included, 

 which embraces what Mr. Walcott assigns to the 

 lower and middle Cambrian divisions. 



It will be noticed that I have here made a series 

 of somewhat arbitrary assumptions on the basis 

 of correlations proposed or implied by good 

 palaeontologists, which correlations are dependent 



on the principle that strata are correctable by 

 means of fossil evidence only. This is done in 

 order to form a temporary link of connection, 

 which may ultimately be dispensed with altogether. 



I have now brought matters into such a position 

 that it is possible to form a standard column for 

 the L. Cambrian, by means of which it can be 

 determined if the fauna of the typical area does 

 or does not coincide with those found in other 

 strata considered to be of the same age by 

 independent evidence. 



The typical area of Wales will first be considered 

 in connection with the formation of a time scale. 

 According to Dr. Hicks the thicknesses and order 

 of succession of the lowest Palaeozoic strata are 

 as follows : 



Feet. 

 L. Arenig : 



Slates and flags .. .. 1,000 



Tremadoc : 



Slates and sandstones .. .. 1,000 



Up. Lingula flags : 



Slates and shales . . . . 600 



Mid. Lingula flags : 



Arenaceous and micaceous flags 2,000 



L. Lingula flags : 



Slates, flags, sandstones and shales 2,500 

 Menevian group : 



Slates and flags . . . . 600 



Longmynd group : 



Harlech grits and Llanberis slates S.coo 



15,700 



The lowest two members he assigns to L. 

 Cambrian ; the four next above to Up. Cambrian, 

 and the highest to L. Silurian. The summit of 

 the Menevian is here considered to mark the latest 

 point of eon 113, as has already been stated. In 

 the case of the Longmynd group it is considered 

 that 1,250 feet of strata represent an eon; and 

 1,000 feet of the flags, slates and shales. 

 According to this calculation the lowest portion is 

 assigned to the pre-Cambrian, that is, it represents 

 the latest portion of the Huronian period of time, 

 the Cambrian commencing at 3,750 feet from the 

 base of the Longmynd group. This would imply 

 that the sea which existed during the Cambrian 

 period first reached the Longmynd area in about 

 eon 119, while the L. Cambrian is supposed to 

 begin with the earliest part of eon 116. With 

 these assumptions the provisional correlation of 

 the strata with time is thus, the first-mentioned 

 being the oldest : 



