426 PROF. J. W. JTJDD ON THE VOLCANIC ROCKS 



large crystals of a nearly colourless pyroxene are scattered some- 

 what sparingly through it. These are certainly, for the most part, 

 monoclinic (augite), though some more decomposed crystals certainly 

 belong to the rhombic pyroxene (enstatite). As a rule, however, the 

 specimens of this rock show only the most incipient stages of altera- 

 tion, the glass and micr elites of the ground-mass exhibiting no sign of 

 change, and many of the porphyritic crystals being as fresh as in a 

 recent lava. Nevertheless, this rock is certainly not of more recent 

 geological age than the Carboniferous, and in all probability it must 

 be referred to the Lower Old Hed Sandstone ! As the augite pre- 

 ponderates so greatly over the enstatite, we may call it an augite- 

 andesite. 



It is an interesting circumstance that the porphyritic crystals of 

 the Northfield rock seldom occur singly, but in groups. In some 

 cases (see Plate XIII. fig. 1) we find aggregates of augite and en- 

 statite crystals with traces of what appears to be decomposed felspar 

 between them. 



The rock of Causewayhead (see Plate XIII. fig. 2), as pointed 

 out by Mr. Durham, is of more highly crystalline type than that of 

 ]N'orthfield ; indeed a glassy base is almost wanting in it, though 

 the felspars are usually very imperfectly individualized ; it has a 

 specific gravity of 2'79. The rock is seen, under the microscope, to 

 be an aggregate of large microlites of triclinic felspar and pyroxene, 

 with some grains of magnetite. The felspar, from its extinction- 

 angles, appears to be near to andesine in composition, and some of 

 the larger and better-defined crystals exhibit the zoning so common 

 in the andesitic type of rocks. The felspars contain enclosures of 

 apatite and other minerals. 



The pyroxene of this rock sometimes occurs in well-formed pris- 

 matic crystals, but there is every gradation from these down to 

 rounded and irregular granules. Although very pale- coloured, the 

 crystals usually exhibit a distinct, though feeble, pleochroism, 

 the tints being those which are characteristic of bronzite or the 

 slightly ferriferous enstatite — a and j3 brownish yellow, y pale green. 

 The intensity of this pleochroism, however, appears to vary in 

 different crystals. The great majority of these pyroxene- crystals 

 give the extinction characteristic of a rhombic mineral ; but I was 

 unable to obtain sufficiently good basal sections to enable me to 

 determine their interference-figures in convergent polarized light. 

 A very striking feature of these pyroxene-crystals is the series of 

 striations and clefts parallel to the base of the prism, which Eosen- 

 busch has so well pointed out as being characteristic of the rhombic 

 pyroxenes*. Although a small portion of the pyroxene in this 

 rock is probably monoclinic — and the mixture of augite and ensta- 

 tite is a common feature in rocks of this class — yet the great 

 majority of the crystals are certainly rhombic. The rock may 

 therefore be classed as an " enstatite-andesite." 



The only important accessory ingredient in the rock is biotite. 

 There are no porphyritic crystals of any kind in the rock, but aggre- 



* Mikroskopische Physiographie, 2nd ed. (1885), p. 393. 



