Pake. — On some Thames Amlesites. 437 



microlites and thinly dusted with magnetite, which also 

 occurs in occasional large irregular aggregates. The feld- 

 spars are plagioclase, translucent, often much clouded with 

 glass inclusions ; not often well developed ; larger pheno- 

 crysts zoned with inclusions and not much twinned ; extinc- 

 tion angles large, indicating a basic variety, probably labra- 

 dorite. Some large plates show no twinning, but exhibit a 

 zoned structure due apparently to a succession of isomorphous 

 layers of growth. A little sanidine is present. The horn- 

 blende is generally altered ; often shows black resorption 

 border ; interior changed to serpentinous matter showing 

 bright polarisation colours ; strongly pleochroic, changing from 

 pale yellowish-brown to greenish-brown. Calcite abundant. 

 With dilute hydrochloric acid brisk effervescence takes place 

 around feldspars and hornblendes. 



Dyke No. 2. — This occurs about a quarter of a mile further 

 south. It runs east and west, and shows a width of about 

 300 ft. It is a compact dark greenish-grey rock. Shows 

 crystals of feldspar and hornblende plainly to unaided eye, 

 but not conspicuously. Under the microscope the ground- 

 mass, or base, is very abundant, clear, and finely dusted with 

 magnetite. The feldspars are zoned with glass inclusions ; in 

 other parts clear and fresh. The inclusions are arranged 

 round the periphery of the crystals, following the crystallo- 

 graphic planes, and also as irregular aggregates along cracks. 

 A little sanidine is present. The hornblendes are mostly 

 altered to serpentinous matter, but do not show resorption 

 borders. Prismatic forms common. Pleochroism ; a = light- 

 brown ; fi and y = dark bluish-green. 



Dyke No. 3. — This occurs about eight chains further south. 

 Its greatest extension appears to be parallel with the coast. 

 It throws out many ramifying branches through the slaty 

 shales ; extends along the beach for nearly 350 yards. This is 

 a dark greenish-grey rock, closely resembling No. 2 in hand 

 samples. Ground-mass clear; not abundant; crowded with 

 feldspar and other mineral microlites. Feldspars mostly clear, 

 well -twinned, showing brilliant polarisation colours; inclu- 

 sions numerous. A few large idiomorphic plates present in 

 each section. A little sanidine present. Hornblendes are all 

 much altered ; outlined with black borders ; centre clear or 

 crowded with decomposition products. Calcite and a little 

 secondary quartz present. A section cut from a sample 

 of this dyke, obtained near southern point of contact with 

 slaty rocks, contains nothing very marked to distinguish it 

 from the section described above, except the condition of the 

 hornblendes, which is less altered, being often quite fresh. 



Dyke No. 4. — This occurs about 45 yards further south. 

 It extends along the beach for some 170 yards. A dark 



