THE GEOLOGY OF THE ENGLISH CHANNEL. 141 



CALC SCHIST. 



M. 14r. S. 2r W. Edd., 20 miles. 



A compact rock with well-marked cleavage, the planes of which are 

 not, however, closely set. Broken across the cleavage, tlie colour is a 

 warm grey and tlie texture close and uniform. The cleavage planes 

 show a somewhat pearly lustre and are stained in parts with red 

 oxide of iron. The rock gives distinct effervescence with cold acid, 

 with warm acid effervesces freely ; fragments retain their form, 

 however, but from the surface a few small quartz grains are set 

 free. 



Tlie section shows numerous clear grains set in granular cement, 

 with which, in places, is much red oxide of iron. Colourless mica 

 (sericite) is rather sparingly developed, being more prominent on the 

 cleavage planes. The clear grains are quartz, with the very rare 

 exception of a felspar, and many show boundaries imposed upon them 

 by the adjacent calcite and dolomite, which freely exhibits the rhombus 

 form of larger or smaller dimensions. The granular cement consists, in 

 fact, almost entirely of minutely crystalline dolomite and calcite, a 

 high power being required to detect the crystal forms. A very large 

 proportion of the quartz grains show secondary enlargement, the 

 secondary quartz having the same crystal axes as the original grain. 

 The boundary between the original and secondary is just such a dark 

 line as occurs when a mineral of greater refractive index is enclosed 

 in a mineral of less. Hair-like microlites are not uncommon in the 

 primary quartz, but none pass over into the marginal secondary 

 growth. In the loose powder obtained by treating this rock in hot 

 acid I found one small crystal of tourmaline. 



The fact that the rock retains its form after treatment with hot acid 

 shows that neither the iron nor the dolomite are necessary cements, 

 the secondary quartz being in itself sufticient. 



Presumedly it is best to call the specimen a calcschist. 



SERPENTINE. 

 M. 24h. 



A small jet-black pebble with very smooth surface. 



The section shows yellowish-green serpentine with "lattice" structure, 

 traversed by roughly parallel streams of dense black material, which 

 also occurs irregularly in cracks of varying direction, and more 

 or less densely diffused in certain parts of the slide over areas which 

 appear reminiscent of the original structure of the rock. The 

 serpentine varies considerably in its depth of shade. At one point 

 it is blue-green around the margin of a clear mineral, which appears 

 possibly to be a plagioclase felspar. 



