366 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 13, 



The whole of the crTstalline rocks, especially the micaceous variety, 

 pass insensibly from the crystalline to the laminated structure. For 

 instance, we have first the granitic base in which the ciystals are 

 somewhat confusedly mixed ; these graduaUy become arranged up- 

 wards into parallel lines, and the rock is then called gneiss ; by de- 

 grees the felspar is decomposed and the mass becomes schistose, with 

 inclosed veins of the predominating element of the compound below, 

 as exhibited in various parts of the section. Such is the general cha- 

 racter of the primary structure of the Andes. 



There are many beds of sandstones, hmestones, &c. covering the 

 above, especially on the eastern chain. The sedimentary formation 

 of Bogota containing coal is very extensive, and is at a very high ele- 

 vation, 8872 feet above the level of the sea. Some of the fossils were 

 presented to the Society in 1843, and described by Professor Forbes 

 in 1 844 * . The flanks are much broken, contorted, and cleaved by 

 the primary stnicture, and many hornblende and quartz veins intrude 

 into them from below. I have seen this effect of the crystalline rocks 

 on mud and clay in old mines ; and also in compact clay-soil m con- 

 tact with schistose rocks. 



The sections having been carefully made, showing in detail the 

 characteristic pheenomena, which are also apphcable to similar rocks 

 in other parts of the world, I must beg reference to them for an ocu- 

 lar demonstration of what I have here briefly attempted to describe. 



Since I made the first sui-vey of the Andes, I have had the oppor- 

 tunity of makhig similar investigations in many other parts ; and 

 subsequently to my last visit, I have geologically and topographically 

 surveyed the whole Isthmus of Panama ; and also many parts of 

 Prussia and other places on this side of the Atlantic ; and lately I 

 have visited the clay-slate districts of AYales ; and the whole, without 

 exception, present an order of structure and transition of the primary 

 base similar to that observed on the ilndes. 



In conclusion, the knowledge of the above phsenomena of order, 

 and of the electro-chemical influence going on within the compound 

 mass, T^ill enable us to determine questions connected vdth mining 

 and unexplored mineral ground, with a degree of certainty hitherto 

 unknown in this most valuable branch of om* industry. Besides, 

 without the knowledge of this important subject, the progress of geo- 

 logical dynamics must be slow and very imperfect, as it is, truly, the 

 key by which we obtain an insight into the cause of the great changes 

 which have occmTcd and still take place on the surface of our globe. 

 These electro-chemical agents are still silently at work, dissolving, re- 

 composing, and cleaving, within the crystalline film that coats the 

 globe ; perpetually mo chfymg the compoimd and rendering it suitable 

 to our wants dming all ages of its transformation, and constantly pro- 

 viding inexhaustible stores of muieral wealth for successive genera- 

 tions. These variations, produced by crystallizing forces, which are 

 more universal, more powerful, and as constant as the upward force 

 of the growth of vegetation, cause changes, which, although insen- 

 sible during the age of man, accumulate and become apparent in long 

 periods of time. 



* Proc. Geol. Soc. vol, iv. p. 391 et seq. 



