Vol. 57.] IGNEOUS KOCKS OF THE TORTWOKTH INLIER. 283 



extinction-angle of less than 10°, though a few give an angle of as much 

 as 40°. There is also a fair number of greatly -altered phenocrysts, 

 which are often zoned by a band of fresher material, and sometimes 

 show marked corrosion by the groundmass. Iron-ores are not at all 

 plentiful. Patches of a brown isotropic and apparently glassy matrix 

 occupy spaces between the felspar-laths. This rock may be called 

 a pyroxene-andesite or basalt. 



The predominant type of rock at Middle Mill Quarry is charac- 

 terized by the abundance of ferric oxide and magnetite, the latter 

 occurring in large irregular patches, and being in all probability not 

 an original constituent. Large amygdules are also very plentiful, 

 and are sometimes filled with calcite, sometimes with a green, often 

 spherulitic chloritic mineral; or, again, the central part of the 

 amj^gdule may be calcite, and the marginal part a chloritic 

 mineral. 



The amygdaloidal rock retains the same general characters in 

 the exposure by the stream south of Middle Mill, but the iron-ores 

 are more uniformly distributed 



(d) Summary with regard to the Igneous Rocks. 



The following are the main characteristics of the trap-rocks : — 

 Felspar-crystals of three types can sometimes be detected, namely : 

 (1) minute needles; (2) abundant laths, which generally appear 

 to be oligoclase or andesine ; and (3) phenocrysts, sometimes of 

 labradorite. Many other rocks show only the laths. 



All the rocks show the presence of bastitic or serpentinous 

 pseudomorphs after pyroxene. Most of this is certainly after 

 a rhombic pyroxene, probably enstatite, but fresh augite is some- 

 times present. We have found no trace of hornblende or of 

 olivine. 



Iron-ore, generally magnetite, is always present as an original 

 constituent, but the amount varies considerably. Ferric oxide, and 

 magnetite apparently of secondary origin, are often present, some- 

 times in considerable quantities. Grains of quartz with corrosion- 

 borders are present in most cases, and constitute one of the most 

 characteristic features of the rocks. 



A small amount of a very fine-grained, or sometimes probably 

 glassy matrix is generally to be detected ; but the rocks, as a 

 whole, are more coarsely crystalline than is usually the case with 

 andesites. Much chlorite and calcite are commonly present, often 

 forming amygdules. 



The silica-percentages obtained (5 8* 16, 58*55, and 63*5 ; or, when 

 raised to a moisture-free basis, 61*44, 63*42, and 67*08) are 

 rather those of andesites than basalts, while the specific gravities 

 (2*74, 2*99) are rather those of basalts. 



On the whole, it may be said that all the rocks belong to the 

 group of pyroxene-andesites or basalts, some being best regarded as 

 pyroxene-audesites, and some as basalts. 



Q. J. G. S. No. 227. x 



