894 F. H. HATCH. 



The Sill, 



The sill consists of a very compact, aphanitic olivine 

 dolerite. It is dark-coloured and has a sub-vitreous appear- 

 ance which almost recalls the characters of Lydian-stone. 

 Under the microscope the rock is seen to be porphyritic, 

 slender, lath-shaped felspars (plagioclase) being embedded in a 

 dark-coloured glassy gi'oundmass, which has undergone only 

 slight devitrification. Occasionally the felspar phenocrysts 

 are aggregated in glomero-porphyritic fashion. In most 

 cases the crystals have the form only of felspar, their 

 felspathic material being replaced by a pseudomorphous 

 mixture of calcite and chlorite. 



The olivine also is only represented by serpentine pseudo- 

 morphs. Some of the grains, however, have preserved their 

 original form, which is quite characteristically that of olivine 

 crystals. 



No oinginal augite could be found, but there are many 

 chlorite pseudoraorphs, which probably represent scattered 

 granules of augite. 



The Contact Zones. 



Towards the actual contact of the two rocks the colour of 

 the groundmass of the dolerite takes on a deeper hue, be- 

 coming finally almost opaque and with no signs of devitrifica- 

 tion. It carries, however, just as many felspar phenocrysts 

 and microlites as before. This darkened zone extends for a 

 distance of five millimetres from the contact, and no doubt 

 represents a "chilled margin." 



Between the actual edge of the dolerite and the unaltered 

 sandstone there is a layer 1 mm. thick, which shows clearly 

 the results of the thermal metamorphism of the dolerite on 

 the sandstone. In this layer the quartz grains appear 

 unchanged both as regards size and position ; but the matrix 

 in which the grains lie is completely modified, the pale car- 



