1850.] DE LA CONDAMINE DISLOCATIONS AT BLACKHEATH. 443 



it has been exposed to the weather. Its colour is caused by the 

 presence of numerous grains of an opake black or dark green sub- 

 stance, the majority of grains being white transparent quartz, with 

 a few yellow grains, and particles of selenite. This bed is often tra- 

 versed by ferruginous strings ; and near the Blackheath Station is 

 intersected by hard black irregular veins, occasionally forming con- 

 centric layers and concretions. This condition may however have 

 been induced after the denudation of the upper beds, exposure to 

 moisture ha^ang frequently produced a deceptive appearance of stra- 

 tification in this bed, as for instance, in the pit in Hyde Vale, Green- 

 wich, and in the cutting east of Lewisham Station. Its surface 

 suffered partial denudation before the deposition of No. 3, as may 

 be seen in the N.E. pit on Loam -pit Hill ; and this condition was 

 strikingly evident in the Blackheath cutting before the sidings were 

 trimmed. Organic remains do not occur, unless we credit the work- 

 men's report that fragile casts of large shells are occasionally found 

 near the base of this stratum at Charlton. 



No. 3 is extremely variable in appearance and composition, though 

 it has some constant characteristics : — 



1 . It always contains more or less green sand or clay. 



2. It always contains pure calcareous matter, in layers, nodules, or 



concretions. 



3. It indicates a less tranquil period than those preceding and suc- 



ceeding it. 



Near Woolwich Arsenal* and at Loam-pit Hill, the base of this bed 

 is full of rolled flint pebbles imbedded in greenish sand ; elsewhere 

 the pebbles are not so numerous, though never entirely absent f. 

 The upper part of this bed best deserves the name of " mottled clay," 

 layers of red, green, yellow, and brown clay and sand succeeding each 

 other in some localities with the greatest irregularity. This was seen 

 especially in the Blackheath cutting, where the "false" or oblique 

 stratification was strongly marked, and caused many slips during the 

 excavation. At Morden College the sand was of a brilliant green 

 hue. At Loam-pit Hill the upper part of the bed abounds in soft 

 nodules of white and cream-coloured friable marl. At Deptford it is 

 partly disseminated in the green clay, and partly concentrated into 

 hard nodules. 



At Charlton and Woolwich the contained calcareous matter (besides 

 forming soft nodules and streaks) has combined with oxide of iron 

 to form hard concretions, some branching, some concentric, and oc- 

 casionally enclosing Cyrence. In the grounds lately occupied by Sir 

 W. Gosset, the concretions form a nearly continuous mass of stone ; 

 there is little or no clay even in the upper part of the bed, and it 

 closely resembles the striped sand No. 5. In Mr. Angerstein's road 

 (Woodlands), where a beautiful section of all the beds has been made, 

 the concretions are large, black, and spheroidal. 



* The denudation of No. 2 here has heen very great. 



t Near the W. Woolwich Station the lower part of this hed becomes hardly 

 distinguishahle from No. 2, although the presence of a few pebhles fixes the line 

 of demarcation. 



VOL. VI. PART I. 2 I 



