164 holland: charnockite series. 



(4) Ultra-basic Division. 

 Pyroxenites. 



Besides the instances of Schlieren described (p. 215) as due 

 to a partial or complete local failure of the plagioclase, pyrox- 

 ene-rocks occur also as narrow dykes cutting through the norites 

 at Pallavaram. In the Pammal hill, two miles west of the railway 

 station, these dykes are found to be from 3 to 5 feet wide,, and are 

 seen distinctly to bifurcate with branches proceeding in different 

 directions through the norite (Nos. 9*394, 9*672). Near the summit of 

 one of the hills on the eastern side of the railway line at Pallavaram 

 there is an exposure of what appears to be a vein 9 inches wide of the 

 non-felspathic rock in a hornblende-norite (No. 9*667). The absence 

 of all signs of chilled edges, such as are distinctly shown for instance 

 by the trap dykes when they cut through the chamockite series 

 shows that the norites were probably still hot when the pyroxene- 

 rocks were intruded, and in the last»mentioned instance, where it 

 is easy to obtain microscopic sections across the junction, the 

 sharpness which the line presents to the naked eye rs not so 

 apparent when the crystals are magnified. Under the microscope 

 the two rocks are seen to differ only by suppression of felspar in 

 that forming the vein ; there is no change in either the size or relative 

 proportion of the ferro-magnesian silicates, and the crystals interlock 

 across the junction line. What, therefore, might be looked upon 

 on account of its limited exposure, as an intrusive vein in the field is 

 really more of the nature of an ultra-basic segregation. As this 

 rock only differs from the norite (No. 9667) in which it occurs, 

 and which has already been described (p. 157), in the absence 

 of felspar, no further details as to its characters need be given. 



The non-felspathic types, however, of Pamma? hill to the west 



of the railway station at Pallavaram form undoubted dykes, and 



on account of their ramifications must be regarded as intrusive in 



the norites. In these cases, also, the intrusions must have taken 



( 46 



