Beviews— Banca and its Tin 8tream-iuorks. 271 



tions, though it must be confessed that a few sections across the 

 country, no matter how rough, would have made the work more 

 complete. 



The Island of Banca consists largely of clay-slate, and sandstone, 

 with intermediate varieties of rock. These rocks contain no fossils, 

 and Van Diest supposes that they are pre-Silurian ; hut as he relies 

 solely on the absence of fossils the opinion is not of very much value. 

 According to the author, these stratified rocks have been broken 

 through by granite, which has altered them into mica-slate, quartzite, 

 and other metamorphic rocks. Tin occurs in Banca in the solid 

 rock and also as stream tin. The former mode of occurrence is 

 summarized as follows : — 



" 1. In Northern Banca tin occurs in the granite in various ways 

 and over a large extent of country. 



"2. The rocks which surround the granite are impregnated with 

 ores and other minerals occurring in'^the granite for some distance, 

 usually not more than a mile and a quarter. 



" 3. These minerals and ores are chiefly deposited in little veins 

 or bunches in the direction of the planes of bedding or in the joints. 



" 4. It is chiefly the sandstone which has taken up these minerals, 

 especially where the rock appears to be the most metamorphosed." 



Van Diest comes to the conclusion that there are no veins of tin 

 ore or tin lodes in Banca which will pay for working. 



With regard to the origin of the tin ore the auihor supposes that 

 the granite was once in a melted state, and contained tin " equably 

 disseminated through it." The granite near the sedimentary roclis 

 cooled quickly, and by its solidification prevented the escape of the 

 metallic particles, though not before some had found their way into 

 the surrounding rocks ; where the granite cooled slowly the heavy 

 metallic particles sank down and were not fixed. 



However simple this theory may be it is hardly quite satisfactory. 

 Every theory accounting for the formation of the tinstone in lodes 

 should at the same time explain the origin of the peculiar minerals 

 which accompany it so universally. Van Diest does not attempt 

 this, and one is tempted to suppose that he is not acquainted with 

 Daubree's masterly essays, where the whole subject of the origin of 

 tin deposits is fully dealt with. 



The description of the mode of deposition of the stream-tin is clear 

 and worthy of attention. The author fully realises the effect of gra- 

 dual weathering and of rain. He does not, however, follow English 

 '' subaerialists" in supposing that the form of the ground is mainly 

 due to subaerial influences ; for in a note (p. 67), after giving an 

 instance of rapid weathering which came under his notice, he says : 

 " From this it may be inferred that the hills when first formed were 

 very much higher and larger and the ravines deeper and steeper 

 then at present." He explains thoroughly the occurrence all along 

 a valley of a bed of tin-ground, i.e. stanniferous gravel. It used to 

 be supposed that this bed had been formed all at once by sudden 

 floods or great rushes of water. The diagram (p. 70) shows very 

 plainly that this tinny gravel or tin-ground must not be looked upon 



