72 DE LA BECHE ON GEOLOGICAL FACTS. 



which faced the north, the direction whence the rock-charged fluid descended ; pro- 

 ceeding down the side valleys, partly occupied by the lower lake of Como, and the lake 

 of Lecco, we find the evidences of such a current in the presence of blocks occurring, 

 as they should do, where direct obstacles were opposed to its course, or in situations 

 where eddies would be produced behind the shoulders of the mountains. One very re- 

 markable instance of such occurrences is behind or on the southern side of Monte San 

 Maurizio, above the town of Como ; where numerous blocks are accumulated on the 

 steep of the mountain, precisely where a body of water, rushing down the great valley, 

 would produce an eddy at its discharge into the open plaius of Italy. The blocks, 

 though no doubt many have descended from their first positions in consequence of the 

 long.continued action of atmospheric agents, occupy an elevated line, as also on other 

 but lower heights in the vicinity, which opposed more direct obstacles to the debacle : 

 seeming to show that the blocks occurred near the surface of the fluid mass, and were 

 whirled by the eddy, at nearly the same level, against the steep sides of this calcareous 

 mountain, as well as thrown against the more direct obstacle of a range of conglome- 

 rate hills."— p. 176. 



We were strongly tempted to give the author's account of the eleva- 

 tion of mountains, and the various instances of this which M. Elie de 

 Beaumont has arranged in a number of systematic groups ; but this we 

 must defer to some future opportunity ; and in the mean time give our 

 readers an opportunity of estimating M. De La Beche's powers of 

 description, in a short article on 



" Granite. 



" In the earlier days of Geology, granite was considered the fundamental rock on 

 which all others accumulated ; but this opinion, like many others, has now given way 

 before facts ; for, as will be seen in the sequel, we have examples of granite, resting 

 upon stratified and fossiliferous rocks of no very great comparative antiquity. It must, 

 however, be confessed, that granite appears sometimes to alternate in considerable thick- 

 ness with the inferior stratified rocks, and that the separation of it from gneiss, parti- 

 cularly thick bedded gneiss, is very ambiguous. Granite is a confused crystalline 

 compound of quartz, felspar, mica, and hornblende. It is not essential that all these 

 four minerals should be present ; on the contrary, rocks have been termed granite when 

 only felspar and mica, felspar and quartz, felspar and hornblende, and quartz and horn- m 

 blende, are the constituent minerals. Such an employment of the term granite must 

 be used with much caution, as, for instance, in the case of the compound of felspar and 

 hornblende, which, in fact, is mineralogical greenstone, and should not be named gra- 

 nite unless it constitutes a very subordinate portion of a mass to which the term may 

 be more properly applied ; and results from the accidental absence of one or two of the 

 above-named minerals for a limited space. The most prevalent compound is one with 

 quartz, felspar, and mica ; when hornblende replaces the mica, it is sometimes called 

 sienite. Other minerals, such as chlorite, talc, steatite, &c, are sometimes arranged 

 with those above enumerated in various ways and proportions ; but such compounds 

 can only be considered as accidental varieties. When the quartz and felspar occur 

 alone, and the crystallization is such that the former appears disseminated in the 

 latter, it is termed graphic granite, from the supposed resemblance it bears to antique 

 characters. Granite is occasionally porphyritie, as is the case iu Cornwall and Devon- 

 shire, large crystals of felspar being disseminated through the mass, showing that 



0 



