22 British Association Reports. 



enabled to trace the progress of events somewhat in detail. We 

 have : — 



1. — The filling in of the east and west joints with red sand, at a 

 period not earlier than, if so early as, the commencement of the 

 Torbay Trias. 



2. — The induration of this sand into coherent and durable dikes 

 capable of being fissured and faulted without their sides falliag in. 



3. — The formation of longitudinal fissures in these dikes. 



4. — The gradual filling up of these fissures, not with sand, but 

 by the precipitation of carbonate of lime. 



5.— The formation of transverse joints, passing in a north and 

 south direction, through the dikes and veins, and the pre-Triassic 

 rocks. 



6. — The faulting of the entire mass, rocks, dikes, and veins, by 

 inequalities of movement in an approximately horizontal direction. 



7. — The filling in of the north and south joints with red sand, 

 as in the first instance, so as to form dikes passing through those 

 previously existing. The two systems being distinguishable by 

 well-defined walls and a marked difference of colour. 



All the events here detailed occurred within the era of the 

 Torbay Trias, and, indeed, apparently during that portion of it in 

 which sandstone alone was deposited, inasmuch as there are no 

 traces of conglomerate in the dikes. 



Before closing this brief communication, I would remark that 

 though I am altogether unprepared to offer any opinion respecting 

 the origin of the joints in question, it appears certain that at least 

 those running north and south cannot be due to contraction in the 

 mass of the rock during its consolidation ; for the rocks themselves 

 are of Middle Devonian Age, between which and the Trias we have 

 the Upper Devonian, the Mountain Limestone, the Millstone Grit, 

 the Coal Measures, and the Permian ages ; so that it is simply im- 

 possible to suj)pose that the rocks had remained unconsolidated. 

 Moreover, the fact that they were already traversed by east and 

 west joints, is a decisive proof that they had assumed the solid 

 condition. 



EI^I'I:ISII jLSSOCiJLTioisr I^:Ex=OI^TS. 



SECTION C— GEOLOGY. 

 I. — On the Existence of Gold-bearing Eruptive Rocks in South America 



WHICH have made THEIR APPEARANCE AT TWO VERY DISTINCT GEOLOGICAL 



periods. By David Forbes, F.R.S., F.G.S., &c. 



THE author believed that the gold deposits of South America had 

 not as yet been studied with a view to determine the geological 

 period at which the gold itself had made its appearance. The 

 present communication was the result of observations made during 

 seven years' travels over a large part of South America, and which 

 had enabled him to class all the deposits of gold which he had 

 visited, under two heads, both of which could be traced directly or 

 indirectly to the intrusion or eruption of igneous rock. 



Under the first head belonged all gold derived from the disinte- 



