RELATION TO MESOZOIC ROCKS IN THE LEPONTINE ALPS. 195 



granules. Indications of mechanical disturbances also are rather 

 conspicuous. 



What explanation are we to give of these markings ? Are they 

 organic or inorganic ? At the first glance they look very like the 

 former. But a tubular mollusc seems to be out of the question, since 

 the forms under the microscope are so very irregular ; further, so 

 far as my experience goes, a calcareous organism generally crystal- 

 lizes more readily and is thus coarser in structure than the matrix. 

 Still this difficulty might perhaps be explained away ; but the former, 

 even allowing for subsequent distortion, seems to me insuperable. 

 But they might be Annelid burrows. Here, however, is the diffi- 

 culty that the matrix is often an almost pure limestone, and the 

 contents of the tube also are calcareous, while the fact that they often 

 " streak away " into the matrix and sometimes are mere " strings " 

 in sections, seems fatal to the hypothesis. After a long and, I hope, 

 unprejudiced examination, I can come to no other conclusion than that 

 these markings are inorganic, and their appearance accords best 

 with the hypothesis that they are the result of local crushing. This 

 may sometimes be due to the accident of the rock being slightly less 

 homogeneous at these places, but I venture to suggest the following 

 as a general explanation. Suppose a pressure acting upon a rock, 

 which if it were mainly composed of parallel folia of a mineral like 

 mica, would produce in them a series of parallel undulations : parallel 

 with the crests of these would be lines of maximum strain or stress 

 which might produce local fracture in the calcite, and when a line of 

 grains once yielded, these would give rise to a linear band of crushed 

 material, which would obviously take a form roughly resembling 

 a flattened-OLit tube. 



The black schistose rock which occurs north of the marble is 

 macroscopically very difficult to determine ; its dominant structure 

 at the present time is obviously due to pressure. Is it a dark mica- 

 ceous schist which has been greatly crushed, or an ordinary shale simi- 

 larly treated ? In other words, is the development of mica, to which 

 its schistose aspect is due, mainly anterior or posterior to the action 

 of the pressure which has caused the present cleavage ? I have 

 seen, as, for instance, in the Tyrol *, in association with white crys- 

 talline limestone a dark mica-schist which has so yielded to pressure 

 as to bear locally a very close resemblance to a phyllite, while the 

 associated marble has been but little affected. Partly owing to the 

 difficulty of obtaining good sections in so fissile a rock, and partly to 

 the destruction of the original mica-flakes in the production of the 

 cleavage, which is followed by a further development of minute 

 filmy mica, a rock in this condition is sometimes locally not easy to 

 distinguish, even under the microscope, from an ordinary phyllite 

 or schistose slate. 



My first impression in the field inclined me to think that this was 

 probably a repetition of the Tyrol case, but a more careful study of 

 the question leads me to the other conclusion. The difficulty of 



* Q.J.G.S. vol. xlv. p. 91. 



