224 Notices of Memoirs — D. Forbes^ 



as a means of identifying the contemporaneous intrusions and out- 

 bursts of the eruptive rocks, which, at different geological epochs, 

 have disturbed the earth's external crust." It was, likewise, observed 

 that " whenever the same mineral is present in two or more rocks of 

 different geological age, it is usually, if not invariably, characterised 

 in each case by certain peculiarities, either in physical structure or 

 chemical composition, which serve to distinguish it under the different 

 circumstances of occurrence." This is shown to be true of felspar, 

 mica, augite, garnet, apatite, hornblende, etc. In short the minerals 

 generally regarded as accidental and extraneous, oftentimes form the 

 real characteristic of the rock-masses containing them. 



The author's labours on this important question have been greatly 

 retarded by the great scarcity of facts which were found available. 

 This is principally due to the neglect of mineralogists to record 

 in their description of minerals their mode of occurrence, their 

 mineral association, and the nature and age of the rock in which 

 they are imbedded, as well as to the want, already mentioned, of 

 chemical analyses of many of even the most common British mineral 

 species. Of the 306 British species and sub-species described in one 

 of the most recently published manuals of mineralogy of the British 

 Islands, 164: have not yet been examined ; and, it would scarcely 

 be believed, that amongst these are included such minerals as horn- 

 blende, augite, orthoclase, labradorite, chlorite, talc, garnet, tourmaline, 

 olivine, epidote, serpentine, beryl, etc. Moreover, our present know- 

 ledge of the composition of British rocks is of an equally unsatisfactory 

 kind. " May it not now be fairly asked whether the natural infer- 

 ence to be deduced from these facts is not, that it is high time for 

 British mineralogists and geologists to set to work, in order to supply 

 these deficiencies before occupying themselves in propounding vague 

 theoretical explanations, to account for the origin and metamorphosis 

 of rocks in the field ?" In the author's papers are given a descrip- 

 tion and analyses of the following British minerals. 



Gold from the Glogau Quartz Lode, No. 2. — The lode occurs in the 

 Lower Silurian Lingula beds, close to their junction with the Cam- 

 brian strata of the Geological Survey ; it runs about 18° north of east, 

 and dips at an angle of 88° to south, cutting through both fossili- 

 ferous strata and the intruded diabases, which are described as green- 

 stones in the Survey ; and it is, consequently, of later geological age 

 than both these rocks, and is not improbably younger than the Silu- 

 rian formation as a whole. The explorations appear to indicate that 

 the lode is more auriferous at the parts where it cuts through the 

 Lingula beds, with their accompanying diabases, than at greater depth 

 where it traverses the Cambrian grits. Among the accessory minerals 

 found in the lode are tetradymite, iron pyrites, chalco-pyrite, galena, 

 chlorite, calcite, dolomite, chalybite, and heavy spar, which, as well 

 as the gold, are distributed very irregularly in the quartz. When 

 the quartz contains calcite, dolomite, and chalybite, or includes frag- 

 ments of neighbouring clay-slate, it is regarded as likely to be more 

 auriferous than when the lode consists of quartz only. When iso- 

 lated fragments of the slate are found in the quartz of the lode, the 



