142 J. HOPKINSON 



there is sufficient evidence to justify this being brought into 

 consideration as at least a probable hypothesis is of interest, in 

 connection with the latest earthquake recorded in our county, 

 and the hurricane and thunderstorm which preceded it. 



The vast majority of earthquakes are doubtless caused by 

 such disturbances of the strata as we find recorded by faults in 

 almost all geological formations. These are tenned tectonic* 



It was the opinion of Prof. John Phillips t that an earthquake 

 happened in Hertfordshire and the valley of the Thames after 

 the Chalk period, but the effect noted, which he tersely states as 

 " pebble beds, wasted surface of chalk," is not decisive evidence. 



We have perhaps better evidence of slight eai-thqiiakes in our 

 county in post-Cretaceous times in the more or less vertical 

 slickensided surfaces of chalk occasionally seen in our chalk-pits ; 

 and in post-Tertiary times in faults in the London Clay and the 

 Eeading Beds, as at Bennett's End near Hemel Hempstead. It 

 is proliable that the anticline in the Chalk passing through 

 Hertfordshire from Reed Hill south of Royston in a south- 

 westerly direction to Pinner in Middlesex and onwards to 

 Windsor was the residt of an earthquake. The movement 

 which gave to the Chalk and overlying Eocene beds the shallow 

 liasin- shaped form from which the district on the north-western 

 margin of which Hertfordshire is situated has been named the 

 London Tertiary basin, must have been too gradual to be 

 perceptible as a shock or series of shocks, although it is believed 

 to have taken place in Miocene times during a period of great 

 seismic activity in the north of Britain, where volcanoes were 

 then in active ei'uption. 



II. Recorded Hertfordshire Earthquakes. 



Coming to historic times there is good reason to believe that 

 many earthquakes which are recorded to have happened in 

 England must have affected Hertfordshire, although we are 

 without local records. These are chiefly such as have been 

 described as of general occurrence all over England, or as 

 affecting localities on two or more sides of the county. In the 

 following enumeration only those are included which have been 

 reported as having been felt, or as having produced visible 

 effects, in Hertfordshire, and the number which I have found 

 recorded, omitting a few evidently erroneous records, is twelve. 



* The theory has, however, recently been advanced by Professor See that all 

 great earthquakes are caused by the percolation of oceanic or surface waters to 

 the molten matter beneath the more solid crust of the earth, and he ascribes the 

 greater frequency of earthquakes in the vicinity of deep coa.«tal -naters to the 

 greater depth and therefore weight of the superincumbent water rather than to 

 the increased weight of the terrestrial .stratn. Prof. See has charge of the 

 U.S. Naval Observatory at Mare Island, California, and studied the San 

 Francisco earthquake. 



t ' Manual of Geology,' p. 573 (1855). 



