1848.] murchison on the structure of the alps. 171 



Lower and Upper Alpine Limestones. 



Liasso- Jurassic and Oxfordian. — Following the classification of 

 M. Lill von Lilienbach, Professor Sedgwick and myself considered 

 the alpine limestone of the Salzburg Alps as divided into two great 

 masses, separated by shale, sandstone and limestone with certain 

 saliferous deposits. In differing from some of our contemporaries in 

 assigning a portion of the lowest of these fossiliferous limestones at 

 Admet, to the age of the lias, I still believe we were correct. But, as 

 before said, in the accurate determination of all the members of these 

 lower secondary rocks, a portion of which is now ascertained to be 

 triassic, so much depends on the researches now in progress by the 

 Austrian palaeontologists, that I forbear to enter upon a further con- 

 sideration of the relations of that tract. Looking on the Alps as a 

 whole, however, we have obtained sufficient proof that the lias and 

 equivalents of the inferior oolite occupy a considerable thickness of 

 the lower portion of what was called alpine limestone. This fact has 

 been clearly established by the presence of fossils in the Venetian, 

 Tyrolese and Milanese Alps. There are indeed tracts in which the 

 Gryphcea incurva and several true liassic ammonites occur in rocks 

 of this age, whilst in Switzerland and the Western Alps the zone 

 has been traced and identified by Studer, JSlie de Beaumont, Sis- 

 monda and many geologists. 



In following these lower Jurassic limestones from the Eastern into 

 the Swiss and Western Alps, great changes in their mineral cha- 

 racter are observable. In the first region they are very frequently 

 light-coloured limestones often in the state of dolomite. In the west 

 they are for the most part dark and even black. As, however, the 

 chief strata in many parts of the Tyrol occur in the form of ordinary 

 limestones, and as these can be followed in the strike until they are 

 found to be transformed into dolomites, so this simple fact seems to 

 me strikingly to corroborate the general view of M. von Buch, that 

 the dolomites of the Alps have been produced by a modification or 

 metamorphism of the original strata. Whatever may have been the 

 proximate cause of this great metamorphosis — w^hether by certain 

 hot vapours or gases, which rose from beneath during one of the 

 revolutions which the chain has undergone, or by any other agent, — 

 it is certain that this cause has acted not only vertically and obliquely, 

 but in many instances horizontally over very large areas ; thus trans- 

 forming the superior strata and leaving comparatively unaltered those 

 beneath. If the crystalline dolomites of the Eastern Alps were the 

 result of original deposit like the magnesian limestone of England, as 

 some geologists aver, then we should not see the irregular and, if I may 

 so say, capricious diffusion of the dolomite, which far from affecting 

 any one set of strata in their horizontal extension, is absent or pre- 

 sent in rocks of various ages and at different horizons. 



Whilst the great masses of dolomite are peculiar to the Eastern 

 Alps, and are most striking in the tracts of the South Tyrol which 

 have been penetrated by porphyries and other igneous rocks, their 

 place is to a great extent taken in the Western Alps by copious masses 



