*S'. Allport — Basalt of South Staffordshire. 115 



The effect of stopping all vegetation and allowing chemico-fluvial 

 denudation to exert its full powers, is well exemplified in the neigh- 

 bourhood of Swansea, where the funies from the copper ore smelting 

 furnaces have changed fruitful fields into a desert, allowing this 

 denudation to act in full force ; and other localities could also be 

 enumerated where its effects may also he traced since the protecting 

 vegetation has been removed. 



V. — On the Basaxt of South Staffordshire. 

 By Samuel Allport, F.G.S. 



AS several papers by Mr. David Forbes and Mr. Sorby have 

 probably by this time induced a few Geologists to turn their 

 attention to the importance of a microscopical examination of rocks 

 and minerals, it may interest some of your readers to know that the 

 application of this method of research has just lead to a rather 

 interesting discovery. 



During the last four years I have made several hundred sections 

 of igneous and metamorphic rocks (chiefly British), and among 

 them a considerable number of the Basalt of the South Staffordshire 

 coal-field. This rock is a Dolerite composed mainly of a triclinic 

 felsjoar, probably Labradorite, Augite and Titano-ferrite. It also fre- 

 quently contains two green minerals, one with a definite crystalline 

 form, the other in streaks, or irregular patches, and occasionally 

 filling cavities ; the latter has every appearance of being a secondary 

 formation, and may be Delessite. In addition to the above, I have 

 found nine or ten other minerals which I will not now stop to 

 enumerate. 



The first discovery due to the microscope was the detection of 

 Olivine. Having observed two small crystals in one of my sections 

 which were evidently different from any of the other minerals, I 

 examined them with polarised light, and found that they presented 

 precisely the same appearances as those seen m sections of Olivine 

 which I had previously made. After many visits to the quarries in 

 search of larger specimens of the mineral, I succeeded in finding it 

 in sufliciently large grains to be easily recognized. 



The green mineral with the crystalline-form above mentioned re- 

 mained to be deteimined. It was evident, in the first place, from 

 its action on polarised light that it was a pseudomorph, the appear- 

 ance of such an aggregate of mineral matter being very different 

 from that of a single uniform crystal ; and, after making more than 

 thirty sections, the last two have enabled me to arrive at the con- 

 clusion that the mineral in question is a pseudomorph after Olivine. 



The two sections contain many crystals of this mineral in various 

 stages of alteration, from the first commencement to the completion 

 of the process. The change began either at the surface, or along 

 the line of cracks running through the crystal ; in the latter case, 

 the transformation extended from both sides of the fissure, fonning 

 a narrow, or wide green band, according as the process continued or 



