Speight. — On a Doleritic Dyke at Dyer's Pass. 411 



crystal seems composed of a kernel round which new material 

 has been deposited (PI. XLIX., fig. 2, and PL XLIXa., fig. 3). 

 In some cases this may be true zonal structure, but sometimes 

 the wave of extinction passes outwards gradually, so that there 

 are no zones at all. The kernel is at times rounded in form, 

 and seemingly corroded, but the resultant shape is idiomorphic. 

 In one case the gradual approximation to the crystallographic 

 outlines could be traced. This structure is not peculiar to the 

 feldspar, but belongs to the augite in a small degree, and 

 appears faintly even in the olivine. There are several facts 

 which show^ that this has been produced late in the history of 

 the crystal : — 



(1.) The kernel is usually irregular in shape, as if it had 

 suffered injury from various solvents, &c. 



(2.) The alteration products and inclusions are usually con- 

 fined to the core, while the periphery is usually free. 



(3.) Cracks in the core suddenly terminate at its edge ; of 

 course, many instances occur in which they are prolonged 

 through the surrounding portion. 



(4.) There is often a zone of alteration products at the edge 

 of the core, as if the crystal had been weathered there and had 

 commenced growing afterwards. 



(5.) Twin lamellse terminate at the edge of the core, though 

 a few sections showed them prolonged further. 



(6.) The periphery is often twinned in a different direction 

 from the interior. 



These observations seem to show conclusively that the 

 crystals had suffered weathering before they were added to. 

 If this is the case the rock must have been solid at the time, 

 and the question is, Where has the new material come from ? 

 If the crystal had been enlarged while the rock was molten 

 it would be easy to understand, but the appearance of 

 weathering renders such an hypothesis improbable. This 

 has been noticed before by Professor Judd [Vide " Quarterly 

 Journal of the Geological Society," vol. xlv., page 175), but 

 in that case the crystals which showed growth were in a glass, 

 and he supposes they grew at its expense under altered condi- 

 tions of temperature and pressure. It would be difficult, 

 however, in this case to account for new growth in this way, 

 since the ground-mass is holocrystalline, and shows no altera- 

 tion in the neighbourhood of the anomalous crystals. These 

 rocks, being from near the surface of a Tertiary volcano, can- 

 not have been buried under subsequent lava-flows, or under 

 sedimentary deposits, so the growing of the crystals cannot 

 have been caused by the influence of changed conditions of 

 temperature and pressure on a glassy ground-mass. Nor can 

 the crystals have been added to by water saturated with 

 feldspathic minerals, forming accretions round minerals re- 



