ON SEISMOLOGICAL INVESTIGATION. 117 



Lower Purbeck, and occui* within a few feet of the Portland Rock. The 

 strike is almost parallel to the fault, though more nearly east and west. 

 Thus it becomes almost certain that Portland Beds crop out at the 

 surface immediately east of the Roman road and are probably within less 

 than 10 feet of the surface at the point where the recording apparatus 

 crosses tlie fault. 



Taking now the trench in which the apparatus is placed, we will 

 describe the strata there seen on each side of the fault. The trench is 

 9 metres long, and at the four observing stations (see Mr. Horace Darwin's 

 Report, p. 119) sections were exposed to a depth varying from 5^ to 7 feet. 

 At Station SS (the southernmost) the depth was 5^ feet, of which the top 

 3 feet was in disturbed ground, the lower 2^ showing hard brownish fine- 

 grained oolite with fossils, the rock being somewhat shattered, with small 

 open fissures, which Avere afterwards filled in with concrete. This rock 

 undoubtedly belongs to the Lower Purbeck ; it seems to dip at a high angle 

 in a southerly direction, the strike, however, not being parallel with the 

 fault. The shallower trench between Stations SS and S showed similar 

 strata, though no fossils or oolitic grains were observed. At Station S the 

 hole was also 5^ feet deep ; the rock being a hard splintery brown limestone, 

 more or less nodular and containing small chert nodules. I believe that this 

 rock corresponds with some cherty limestones which are seen in the large 

 Upway Quarry, just below the 'dirt-bed' and within 5 or 10 feet of the 

 base of the Purbecks. Near the fault, however, they are harder and 

 more crystalline than in the quarry. I was not able to find the earthy 

 and carbonaceous ' dirt-bed ' at this point, though it is so well seen only 

 50 or 60 feet away (see fig. 5). The squeezing-out or thickening of a soft 

 stratum is, however, a phenomenon constantly to be met with near a big 

 distux'bance, and the absence of the cai'bonaceous seam is probably due to 

 this cause. The south cheek of the fault consists of brecciated white 

 limestone with chert. These exposures seem to indicate that the Portland 

 Stone must occur within 5 feet or so of the surface close to the fault, and 

 on the strength of the new evidence I have added an inlier of Portland 

 rock to the map made by Mr. Strahan, Avho agrees with me that such an 

 addition is necessary. 



The fault itself is represented by a band of fault-rock not more than 

 2 feet in thickness and quite unlike the wide dyke of mingled Oolite and 

 Oxford Clay seen in the railway-cutting. In our trench the fault-rock is 

 a hard mass of breccia consisting of Upper Chalk and fragments of 

 Purbeck Limestone. 



The north cheek of the fault consists of very hard shattered and 

 re-crystallised flinty chalk like that associated with the similar disturb- 

 ances at Corfe Castle and at Ballard Clifi", though at Ridgeway I did not 

 observe actual calcite veins. Two feet north of the fault I dug out a 

 specimen of Anancliytes ovatus ; but this echinoderm and a few fragments 

 of Inoceranuis were the only fossils I could find in the Chalk in our trench. 

 The flinty character of the Chalk and the presence of the Anancliytes 

 show, however, that we have passed suddenly from Lower Purbeck to Upper 

 Chalk, and the character of the Chalk and of the included flints indicates, I 

 think, that we are at an horizon above the Micraster-zones and probably at 

 least 300 feet above the base of the Chalk. 



Between the fault and Station N the Chalk gradually becomes softer 

 and less crystalline and contains small broken flints, black with moderate 

 rinds. The hole at Station NN showed 6 feet of moderately hard Chalk, 



