Forbes — TJie Microscope in Geology. olo 



2. Eocks built up of the more or less rounded or angular debris 



of previously existing sedimentary or eruptive rocks. 



3. Eocks composed of mineral substances extracted from aqueous 



solution by crystallisation, precipitation, or the action of 

 organic life. 



1. Rocks composed of the immediate products of the hr caking -up of 

 eruptive rocks. — The little attention paid by geologists in general to 

 the study of rocks of this class, has introduced the elements of con- 

 fusion into many of their enquiries, and frequently has led to very 

 erroneous opinions being formed as to the nature and origin of 

 certain rocks, which could never have been entertained had micro- 

 scopic investigation gone hand in hand with field observation. 



Eocks of this class may either be of subaerial or subaqueous 

 origin ; in the former case, for example, volcanic ashes may have 

 been deposited as beds on the surface of the land, and afterwards 

 been covered by lava streams poured out over them ; or, from having 

 been depressed below the sea level, may have had sedimentary beds 

 of aqueous origin subsequently superposed on them. 



When of subaqueoiis origin, as is by far the most common case, 

 subaerial or subaqueous outbursts may force into the sea eruptive 

 rocks, which, being at once broken up into a state of division more 

 or less fine, in proportion to the greater or lesser cooling power of 

 the watery mass in immediate contact, may be spread out into beds 

 by the action of the waves : the texture of these rocks may vary 

 from that of the coarsest breccia down to the finest mud, and, as is 

 usually the case, such deposits may present themselves as alternating 

 beds of coarse and fine character. Upon the consolidation of such 

 formations, rocks are formed, identical in chemical and mineralogical 

 composition with the original eruptive rock from which they were 

 derived, and which, particularly when close-graiaed, often present an 

 external appearance so like the original rocks as to be frequently 

 undistinguishable from them by the naked eye ; in such deposits it 

 is often easy to pick out specimens having all gradations in appear- 

 ance from the above described down to such as would be attributed 

 to the consolidation of mere detrital mud. 



No wonder, therefore, if the field geologist finds himself bewildered 

 under such circumstances, and inclined to settle down in the comfort- 

 able belief of the transmutation or transition of sedimentary rocks 

 into eruptive, etc. ; even the chemist feels puzzled, when he finds 

 that a rock taken out of apparently normal stratified deposits has the 

 same chemical composition with one of undoubtedly intrusive nature. 

 The microscopic examination, however, soon shows that, however 

 similar the external appearance of two such rocks might be, their 

 internal structure is totally different ; showing in the primary rock 

 the crystallised structure and arrangement previously described, 

 whilst the secondary rock is resolved into a mere agglomeration of 

 more or less broken fragments of the same minerals constituting the 

 former. In beds formed from the consolidation of volcanic ashes, 

 the microscopic examination occasionally affords evidence as to 

 whether such ashes had been deposited on land, or had fallen into 

 water. 



