C— GEOLOGY. 73 



showing the relative chronological position of the numerous igneous rocks 

 which are intrusive into them, or interbedded with them, as lava flows 

 and tuffs (volcanic ashes) ; (4) typical fossils, characteristic of the main 

 divisions of geological time, particularly those which are associated with 

 economic minerals, such as coal, brown coal, lignite, oil-shale and mineral 

 oil ; (5) typical fuels ; (6) the common metallic and noti-metallic minerals 

 (as far as possible these specimens should not be only of the beautiful, 

 showy type that are seen in great museums, but also of the weather-beaten 

 ones, such as one finds at or near the surface, during the course of 

 prospecting for minerals) ; (7) gemstones ; (8) clays, sands, gravels, 

 pigments, abrasives, refractories ; (9) building, ornamental and engineering 

 stones (with a section illustrating the various kinds of limestone and their 

 products) ; (10) a collection showing the natural weathering of rocks ; 

 (11) concentrates, and their minerals ; (12) a collection, specially for use 

 in agriculture and forestry, and in assistance in the proper determination 

 and mode of occurrence of rocks, the nature of which is more or less 

 obscure. This should comprise groups of five or six specimens repre- 

 senting : — - 



(a) Fresh (undecayed), or non-disintegrating rock ; (b) partially 

 decayed, or disintegrating one, showing a crust of decayed rock round a 

 <;ore of fresh rock ; (c) completely decayed rock ; (d) subsoil from (c) ; 

 (e) soil immediately above the subsoil ; (/) soil mixed with humus. 



A separate small collection of the typical rocks and minerals of the 

 colony should also be shown. 



Since many rocks decay into soils which in colour differ greatly from 

 the fresh or decaying rocks, it can be seen that a study of examples of this 

 kind enables one to form shrewd conclusions regarding the rocks from 

 which the soils have been derived, especially in tropical countries where 

 decay has been so great — to upwards of 50 feet — that no rock, fresh or 

 decaying, can be seen in natural sections, such as low cliffs, channels and 

 landslips ; or artificial ones, as railway- and road-cuttings, drains, shafts, 

 and pits. Thus the soil can be determined as a sedentary one — derived 

 directly from the rock underlying — or one of transport, consisting of 

 material usually quite different from a soil derivable from the rocks 

 underlying. One useful aid to determination of a sedentary soil is the 

 presence of lines of quartz representing disintegrated veins of that mineral. 

 To appreciate fully the importance of specimens of this kind it is advisable 

 to compare a series of groups of them. 



Accompanying these specimens there should be a diagram of transverse 

 sections showing in profile the nature of the weathering from the surface 

 soil to the underlying rock at the base, the chemical and physical analyses 

 of the soil, the nature of the drainage, &c. 



Photographs and diagrams, showing various aspects and phases of all 

 occurrences, with descriptive and explanatory notes, should be put with 

 the specimens. 



Gold Coast Method of Kapid Survey in Unmapped Country. 

 It may be advisable to describe very briefly the main features of the 

 method adopted in the Gold Coast in the rapid examination of country 

 possessing no reliable maps. 



