600 MESSRS. S. H. REYNOLDS AND C. I. GARDINER [Nov. 1 896, 



VII. The Hill oe Allen. 



At the present time no sedimentary rocks are exposed on the 

 Hill of Allen, and the succession of the igneous rocks is by no means 

 easy to make out. There are several well-marked bands of rock 

 which strike roughly north-east and south-west, but the direction 

 of their dip is not very clear. 



The ashes and andesites seen at the base of the igneous rocks of 

 Grange Hill have no representatives exposed on the Hill of Allen, 

 where the lowest rock seen is that at the base of the south-eastern 

 face, and is a dark vesicular basalt, generally characterized by the 

 presence of porphyritic augites. Under the microscope, porphyritic 

 felspars are to be seen as well ; these are, however, nearly always 

 greatly altered, though the alteration-products are commonly rimmed 

 with fresh secondary felspar. In a rock from the spot marked 60 

 on the map (tig. 1, p. 588) many of the felspars are very fresh and 

 well twinned. 



Eesting on this comes another basalt, fine in texture, showing no 

 porphyritic constituents in a hand-specimen, and generally some- 

 what red in colour, though occasionally green. This rock is often 

 very vesicular, the vesicles being filled either with calcite or chlorite. 

 Some sections show a groundmass deeply stained with haematite 

 and containing many small felspars. The porphyritic constituents 

 include fair-sized plagioclase-felspars with their margins much 

 fretted and their substance entirely replaced by alteration-products. 

 Large fresh augites occur aggregated together, and there is present, 

 as on Grange Hill, a mineral in the form of shapeless grains often 

 crossed with curved cracks. These grains are generally surrounded 

 by a deep red-brown rim of haematite which also fills the cracks, 

 while the spaces between the cracks are now often occupied by 

 serpentine. There seems every probability that these were originally 

 grains of olivine. Grains of chalybite are often found quite similar 

 to those in the rocks of Grange Hill. 



These basalts are, we think, to be correlated with those which 

 immediately overlie the lowest andesitic flows of Grange Hill. 

 Amongst both we find rocks sometimes dark green, sometimes red, 

 showing sometimes porphyritic augites, sometimes porphyritic 

 felspars. Sections of these rocks from both hills show augite- 

 crystals aggregated together, and porphyritic felspars which have 

 been entirely replaced by alteration-products and afterwards rimmed 

 with fresh felspathic material. The occurrence of chalybite, and of 

 pseudomorphs after olivine, is also characteristic of both hills. 



On the Hill of Allen, as on Grange Hill, these basalts are over- 

 lain by a coarse porphyritic basalt, the junction between the two 

 being in some places very sharp and in others more or less irregular. 

 At some points there seem to be portions of the fine basalt caught 

 up in the coarser rock, and this is particularly noticeable at a point 

 east of the tower on the top of the hill, where a well-marked but 

 very local breccia occurs. This breccia consists of blocks of both 

 the coarse and fine rocks mingled together. The blocks and frag- 



